diff --git a/test/membership/group_database_constraints_test.exs b/test/membership/group_database_constraints_test.exs
new file mode 100644
index 0000000..4418b33
--- /dev/null
+++ b/test/membership/group_database_constraints_test.exs
@@ -0,0 +1,141 @@
+defmodule Mv.Membership.GroupDatabaseConstraintsTest do
+  @moduledoc """
+  Tests for database-level constraints (unique, foreign keys, CASCADE).
+  These tests verify that constraints are enforced at the database level, not just application level.
+  """
+  use Mv.DataCase, async: false
+
+  alias Mv.Membership
+
+  require Ash.Query
+  import Ash.Expr
+
+  setup do
+    system_actor = Mv.Helpers.SystemActor.get_system_actor()
+    %{actor: system_actor}
+  end
+
+  describe "Unique Constraints" do
+    test "database enforces unique name constraint (case-insensitive via LOWER)", %{actor: actor} do
+      {:ok, _group1} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      # Try to create with same name, different case - should fail at DB level
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_group(%{name: "TEST GROUP"}, actor: actor)
+
+      assert Enum.any?(errors, fn err ->
+               err.field == :name and
+                 (String.contains?(err.message, "already been taken") or
+                    String.contains?(err.message, "already exists") or
+                    String.contains?(err.message, "duplicate"))
+             end)
+    end
+
+    test "database enforces unique slug constraint (case-sensitive)", %{actor: actor} do
+      {:ok, _group1} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      # Try to create with name that generates same slug - should fail at DB level
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_group(%{name: "test-group"}, actor: actor)
+
+      assert Enum.any?(errors, fn err ->
+               (err.field == :slug or err.field == :name) and
+                 (String.contains?(err.message, "already been taken") or
+                    String.contains?(err.message, "already exists") or
+                    String.contains?(err.message, "duplicate"))
+             end)
+    end
+  end
+
+  describe "Foreign Key Constraints" do
+    test "cannot create MemberGroup with non-existent member_id", %{actor: actor} do
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+      fake_member_id = Ash.UUID.generate()
+
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_member_group(%{member_id: fake_member_id, group_id: group.id},
+                 actor: actor
+               )
+
+      assert Enum.any?(errors, fn err ->
+               (err.field == :member_id or err.field == :member) and
+                 (String.contains?(err.message, "does not exist") or
+                    String.contains?(err.message, "not found") or
+                    String.contains?(err.message, "foreign key"))
+             end)
+    end
+
+    test "cannot create MemberGroup with non-existent group_id", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      fake_group_id = Ash.UUID.generate()
+
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_member_group(%{member_id: member.id, group_id: fake_group_id},
+                 actor: actor
+               )
+
+      assert Enum.any?(errors, fn err ->
+               (err.field == :group_id or err.field == :group) and
+                 (String.contains?(err.message, "does not exist") or
+                    String.contains?(err.message, "not found") or
+                    String.contains?(err.message, "foreign key"))
+             end)
+    end
+  end
+
+  describe "CASCADE Delete Constraints" do
+    test "deleting member cascades to member_groups (verified at DB level)", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      {:ok, member_group} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Verify association exists
+      assert member_group.member_id == member.id
+
+      # Delete member
+      :ok = Membership.destroy_member(member, actor: actor)
+
+      # Verify MemberGroup is deleted at DB level (CASCADE)
+      {:ok, mgs} =
+        Ash.read(
+          Mv.Membership.MemberGroup
+          |> Ash.Query.filter(expr(id == ^member_group.id)),
+          actor: actor,
+          domain: Mv.Membership
+        )
+
+      assert mgs == []
+    end
+
+    test "deleting group cascades to member_groups (verified at DB level)", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      {:ok, member_group} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Verify association exists
+      assert member_group.group_id == group.id
+
+      # Delete group
+      :ok = Membership.destroy_group(group, actor: actor)
+
+      # Verify MemberGroup is deleted at DB level (CASCADE)
+      {:ok, mgs} =
+        Ash.read(
+          Mv.Membership.MemberGroup
+          |> Ash.Query.filter(expr(id == ^member_group.id)),
+          actor: actor,
+          domain: Mv.Membership
+        )
+
+      assert mgs == []
+    end
+  end
+end
diff --git a/test/membership/group_integration_test.exs b/test/membership/group_integration_test.exs
new file mode 100644
index 0000000..7664660
--- /dev/null
+++ b/test/membership/group_integration_test.exs
@@ -0,0 +1,141 @@
+defmodule Mv.Membership.GroupIntegrationTest do
+  @moduledoc """
+  Integration tests for many-to-many relationships and query performance.
+  """
+  use Mv.DataCase, async: false
+
+  alias Mv.Membership
+
+  require Ash.Query
+  import Ash.Expr
+
+  setup do
+    system_actor = Mv.Helpers.SystemActor.get_system_actor()
+    %{actor: system_actor}
+  end
+
+  describe "Many-to-Many Relationship" do
+    test "member can belong to multiple groups", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group1} = Membership.create_group(%{name: "Group One"}, actor: actor)
+      {:ok, group2} = Membership.create_group(%{name: "Group Two"}, actor: actor)
+      {:ok, group3} = Membership.create_group(%{name: "Group Three"}, actor: actor)
+
+      # Add member to all groups
+      {:ok, _mg1} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group1.id},
+          actor: actor
+        )
+
+      {:ok, _mg2} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group2.id},
+          actor: actor
+        )
+
+      {:ok, _mg3} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group3.id},
+          actor: actor
+        )
+
+      # Load member with groups
+      {:ok, member_with_groups} =
+        Ash.load(member, :groups, actor: actor, domain: Mv.Membership)
+
+      assert length(member_with_groups.groups) == 3
+      assert Enum.any?(member_with_groups.groups, &(&1.id == group1.id))
+      assert Enum.any?(member_with_groups.groups, &(&1.id == group2.id))
+      assert Enum.any?(member_with_groups.groups, &(&1.id == group3.id))
+    end
+
+    test "group can contain multiple members", %{actor: actor} do
+      {:ok, member1} = Membership.create_member(%{email: "member1@test.com"}, actor: actor)
+      {:ok, member2} = Membership.create_member(%{email: "member2@test.com"}, actor: actor)
+      {:ok, member3} = Membership.create_member(%{email: "member3@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      # Add all members to group
+      {:ok, _mg1} =
+        Membership.create_member_group(%{member_id: member1.id, group_id: group.id},
+          actor: actor
+        )
+
+      {:ok, _mg2} =
+        Membership.create_member_group(%{member_id: member2.id, group_id: group.id},
+          actor: actor
+        )
+
+      {:ok, _mg3} =
+        Membership.create_member_group(%{member_id: member3.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Load group with members
+      {:ok, group_with_members} =
+        Ash.load(group, :members, actor: actor, domain: Mv.Membership)
+
+      assert length(group_with_members.members) == 3
+      assert Enum.any?(group_with_members.members, &(&1.id == member1.id))
+      assert Enum.any?(group_with_members.members, &(&1.id == member2.id))
+      assert Enum.any?(group_with_members.members, &(&1.id == member3.id))
+    end
+  end
+
+  describe "Query Performance" do
+    test "preloading groups with members avoids N+1 queries", %{actor: actor} do
+      # Create test data
+      {:ok, member1} = Membership.create_member(%{email: "member1@test.com"}, actor: actor)
+      {:ok, member2} = Membership.create_member(%{email: "member2@test.com"}, actor: actor)
+      {:ok, group1} = Membership.create_group(%{name: "Group One"}, actor: actor)
+      {:ok, group2} = Membership.create_group(%{name: "Group Two"}, actor: actor)
+
+      # Create associations
+      {:ok, _mg1} =
+        Membership.create_member_group(%{member_id: member1.id, group_id: group1.id},
+          actor: actor
+        )
+
+      {:ok, _mg2} =
+        Membership.create_member_group(%{member_id: member1.id, group_id: group2.id},
+          actor: actor
+        )
+
+      {:ok, _mg3} =
+        Membership.create_member_group(%{member_id: member2.id, group_id: group1.id},
+          actor: actor
+        )
+
+      # Count queries using Telemetry
+      query_count = Agent.start_link(fn -> 0 end) |> elem(1)
+
+      handler = fn _event, _measurements, _metadata, _config ->
+        Agent.update(query_count, &(&1 + 1))
+      end
+
+      :telemetry.attach("test-query-counter", [:ash, :query, :start], handler, nil)
+
+      # Load all members with groups preloaded (should be efficient with JOIN)
+      {:ok, members} =
+        Ash.read(Mv.Membership.Member, actor: actor, domain: Mv.Membership, load: [:groups])
+
+      final_count = Agent.get(query_count, & &1)
+      :telemetry.detach("test-query-counter")
+
+      member1_loaded = Enum.find(members, &(&1.id == member1.id))
+      member2_loaded = Enum.find(members, &(&1.id == member2.id))
+
+      # Verify preloading worked
+      assert length(member1_loaded.groups) == 2
+      assert length(member2_loaded.groups) == 1
+
+      # Verify groups are correctly associated
+      assert Enum.any?(member1_loaded.groups, &(&1.id == group1.id))
+      assert Enum.any?(member1_loaded.groups, &(&1.id == group2.id))
+      assert Enum.any?(member2_loaded.groups, &(&1.id == group1.id))
+
+      # Verify query count is reasonable (should be 2 queries: one for members, one for groups)
+      # Note: Exact count may vary based on Ash implementation, but should be much less than N+1
+      assert final_count <= 3,
+             "Expected max 3 queries (members + groups + possible count), got #{final_count}. This suggests N+1 query problem."
+    end
+  end
+end
diff --git a/test/membership/group_test.exs b/test/membership/group_test.exs
new file mode 100644
index 0000000..a9e058a
--- /dev/null
+++ b/test/membership/group_test.exs
@@ -0,0 +1,284 @@
+defmodule Mv.Membership.GroupTest do
+  @moduledoc """
+  Tests for Group resource validations, CRUD operations, and relationships.
+  """
+  use Mv.DataCase, async: true
+
+  alias Mv.Membership
+
+  require Ash.Query
+  import Ash.Expr
+
+  setup do
+    system_actor = Mv.Helpers.SystemActor.get_system_actor()
+    %{actor: system_actor}
+  end
+
+  describe "Validations - Name & Description" do
+    @valid_attrs %{
+      name: "Test Group",
+      description: "Test description"
+    }
+
+    test "create group with valid attributes", %{actor: actor} do
+      assert {:ok, group} = Membership.create_group(@valid_attrs, actor: actor)
+      assert group.name == "Test Group"
+      assert group.description == "Test description"
+      assert group.slug != nil
+    end
+
+    test "create group with name only (description nil)", %{actor: actor} do
+      attrs = Map.delete(@valid_attrs, :description)
+      assert {:ok, group} = Membership.create_group(attrs, actor: actor)
+      assert group.name == "Test Group"
+      assert group.description == nil
+    end
+
+    test "return error when name is missing", %{actor: actor} do
+      attrs = Map.delete(@valid_attrs, :name)
+
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_group(attrs, actor: actor)
+
+      assert error_message(errors, :name) =~ "must be present"
+    end
+
+    test "return error when name exceeds 100 characters", %{actor: actor} do
+      long_name = String.duplicate("a", 101)
+      attrs = Map.put(@valid_attrs, :name, long_name)
+
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_group(attrs, actor: actor)
+
+      assert error_message(errors, :name) =~ "must be at most 100"
+    end
+
+    test "return error when name is not unique (case-insensitive) - application level validation",
+         %{
+           actor: actor
+         } do
+      {:ok, _group1} = Membership.create_group(@valid_attrs, actor: actor)
+
+      # Try to create with same name, different case
+      # This tests application-level validation (Ash validations)
+      attrs2 = Map.put(@valid_attrs, :name, "TEST GROUP")
+
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_group(attrs2, actor: actor)
+
+      error_msg = error_message(errors, :name)
+      assert error_msg =~ "already been taken" || error_msg =~ "already exists"
+    end
+
+    test "description max length is 500 characters", %{actor: actor} do
+      long_description = String.duplicate("a", 501)
+      attrs = Map.put(@valid_attrs, :description, long_description)
+
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_group(attrs, actor: actor)
+
+      assert error_message(errors, :description) =~ "must be at most 500"
+    end
+  end
+
+  describe "Slug Generation & Validation" do
+    test "slug is automatically generated from name on create", %{actor: actor} do
+      {:ok, group} =
+        Membership.create_group(%{name: "Test Group Name"}, actor: actor)
+
+      assert group.slug == "test-group-name"
+    end
+
+    test "slug is unique (prevents duplicate slugs from different names)", %{actor: actor} do
+      {:ok, _group1} =
+        Membership.create_group(%{name: "Test!!!"}, actor: actor)
+
+      # Second group with name that generates same slug should fail
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_group(%{name: "Test???"}, actor: actor)
+
+      assert Enum.any?(errors, fn err ->
+               (err.field == :slug or err.field == :name) and
+                 (String.contains?(err.message, "already been taken") or
+                    String.contains?(err.message, "already exists"))
+             end)
+    end
+
+    test "slug is immutable (doesn't change when name is updated)", %{actor: actor} do
+      {:ok, group} =
+        Membership.create_group(%{name: "Original Name"}, actor: actor)
+
+      original_slug = group.slug
+      assert original_slug == "original-name"
+
+      {:ok, updated_group} =
+        Membership.update_group(group, %{name: "New Different Name"}, actor: actor)
+
+      assert updated_group.slug == original_slug
+      assert updated_group.name == "New Different Name"
+    end
+
+    test "slug cannot be empty (rejects name with only special characters)", %{actor: actor} do
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_group(%{name: "!!!"}, actor: actor)
+
+      assert Enum.any?(errors, fn err ->
+               (err.field == :slug or err.field == :name) and
+                 (String.contains?(err.message, "cannot be empty") or
+                    String.contains?(err.message, "is required"))
+             end)
+    end
+  end
+
+  describe "CRUD Operations" do
+    test "create group with name and description", %{actor: actor} do
+      attrs = %{name: "New Group", description: "Description"}
+
+      assert {:ok, group} = Membership.create_group(attrs, actor: actor)
+      assert group.name == "New Group"
+      assert group.description == "Description"
+    end
+
+    test "update group name (slug remains unchanged)", %{actor: actor} do
+      {:ok, group} = Membership.create_group(%{name: "Original"}, actor: actor)
+      original_slug = group.slug
+
+      {:ok, updated} = Membership.update_group(group, %{name: "Updated"}, actor: actor)
+
+      assert updated.name == "Updated"
+      assert updated.slug == original_slug
+    end
+
+    test "update group description", %{actor: actor} do
+      {:ok, group} =
+        Membership.create_group(%{name: "Test", description: "Old"}, actor: actor)
+
+      {:ok, updated} =
+        Membership.update_group(group, %{description: "New Description"}, actor: actor)
+
+      assert updated.description == "New Description"
+    end
+
+    test "prevent duplicate name on update (case-insensitive)", %{actor: actor} do
+      {:ok, _group1} = Membership.create_group(%{name: "Group One"}, actor: actor)
+      {:ok, group2} = Membership.create_group(%{name: "Group Two"}, actor: actor)
+
+      # Try to update group2 with group1's name (different case)
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.update_group(group2, %{name: "GROUP ONE"}, actor: actor)
+
+      error_msg = error_message(errors, :name)
+      assert error_msg =~ "already been taken" || error_msg =~ "already exists"
+    end
+  end
+
+  describe "Calculations" do
+    test "member count calculation returns 0 for empty group", %{actor: actor} do
+      {:ok, group} = Membership.create_group(%{name: "Empty Group"}, actor: actor)
+
+      # Load with calculation
+      {:ok, group_with_count} =
+        Ash.load(group, :member_count, actor: actor, domain: Mv.Membership)
+
+      assert group_with_count.member_count == 0
+    end
+
+    test "member count calculation returns correct count when members added/removed", %{
+      actor: actor
+    } do
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+      {:ok, member1} = Membership.create_member(%{email: "member1@test.com"}, actor: actor)
+      {:ok, member2} = Membership.create_member(%{email: "member2@test.com"}, actor: actor)
+
+      # Add members to group
+      {:ok, _mg1} =
+        Membership.create_member_group(%{member_id: member1.id, group_id: group.id},
+          actor: actor
+        )
+
+      {:ok, _mg2} =
+        Membership.create_member_group(%{member_id: member2.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Check count
+      {:ok, group_with_count} =
+        Ash.load(group, :member_count, actor: actor, domain: Mv.Membership)
+
+      assert group_with_count.member_count == 2
+
+      # Remove one member
+      {:ok, mg_to_delete} =
+        Ash.read_one(
+          Mv.Membership.MemberGroup
+          |> Ash.Query.filter(expr(member_id == ^member1.id and group_id == ^group.id)),
+          actor: actor,
+          domain: Mv.Membership
+        )
+
+      :ok = Membership.destroy_member_group(mg_to_delete, actor: actor)
+
+      # Check count again
+      {:ok, group_with_count_updated} =
+        Ash.load(group, :member_count, actor: actor, domain: Mv.Membership)
+
+      assert group_with_count_updated.member_count == 1
+    end
+  end
+
+  describe "Relationships & Deletion" do
+    test "group has many_to_many members relationship (load with preloading)", %{actor: actor} do
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+
+      {:ok, _mg} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Load group with members
+      {:ok, group_with_members} =
+        Ash.load(group, :members, actor: actor, domain: Mv.Membership)
+
+      assert length(group_with_members.members) == 1
+      assert hd(group_with_members.members).id == member.id
+    end
+
+    test "delete group cascades to member_groups (members remain intact)", %{actor: actor} do
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+
+      {:ok, _mg} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Delete group
+      :ok = Membership.destroy_group(group, actor: actor)
+
+      # Member should still exist
+      {:ok, member_reloaded} = Ash.get(Mv.Membership.Member, member.id, actor: actor)
+      assert member_reloaded != nil
+
+      # MemberGroup should be deleted
+      {:ok, mgs} =
+        Ash.read(
+          Mv.Membership.MemberGroup
+          |> Ash.Query.filter(expr(group_id == ^group.id)),
+          actor: actor,
+          domain: Mv.Membership
+        )
+
+      assert mgs == []
+    end
+  end
+
+  # Helper function for error evaluation
+  # Returns the error message for a given field, or empty string if not found
+  defp error_message(errors, field) do
+    case Enum.find(errors, fn err -> Map.get(err, :field) == field end) do
+      nil -> ""
+      err -> Map.get(err, :message, "")
+    end
+  end
+end
diff --git a/test/membership/member_group_test.exs b/test/membership/member_group_test.exs
new file mode 100644
index 0000000..54f9ff9
--- /dev/null
+++ b/test/membership/member_group_test.exs
@@ -0,0 +1,116 @@
+defmodule Mv.Membership.MemberGroupTest do
+  @moduledoc """
+  Tests for MemberGroup join table resource - validations and cascade delete behavior.
+  """
+  use Mv.DataCase, async: true
+
+  alias Mv.Membership
+
+  require Ash.Query
+  import Ash.Expr
+
+  setup do
+    system_actor = Mv.Helpers.SystemActor.get_system_actor()
+    %{actor: system_actor}
+  end
+
+  describe "Validations & Associations" do
+    test "create association between member and group", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      assert {:ok, member_group} =
+               Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+                 actor: actor
+               )
+
+      assert member_group.member_id == member.id
+      assert member_group.group_id == group.id
+    end
+
+    test "prevent duplicate associations (same member + same group)", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      {:ok, _mg1} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Try to create duplicate
+      assert {:error, %Ash.Error.Invalid{errors: errors}} =
+               Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+                 actor: actor
+               )
+
+      assert Enum.any?(errors, fn err ->
+               ((err.field == :member_id or err.field == :group_id) and
+                  String.contains?(err.message, "already been taken")) or
+                 String.contains?(err.message, "already exists") or
+                 String.contains?(err.message, "duplicate")
+             end)
+    end
+  end
+
+  describe "Cascade Delete Behavior" do
+    test "cascade delete when member deleted (MemberGroup deleted, Group remains)", %{
+      actor: actor
+    } do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      {:ok, _mg} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Delete member
+      :ok = Membership.destroy_member(member, actor: actor)
+
+      # Group should still exist
+      {:ok, group_reloaded} = Ash.get(Mv.Membership.Group, group.id, actor: actor)
+      assert group_reloaded != nil
+
+      # MemberGroup should be deleted
+      {:ok, mgs} =
+        Ash.read(
+          Mv.Membership.MemberGroup
+          |> Ash.Query.filter(expr(member_id == ^member.id)),
+          actor: actor,
+          domain: Mv.Membership
+        )
+
+      assert mgs == []
+    end
+
+    test "cascade delete when group deleted (MemberGroup deleted, Member remains)", %{
+      actor: actor
+    } do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      {:ok, _mg} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Delete group
+      :ok = Membership.destroy_group(group, actor: actor)
+
+      # Member should still exist
+      {:ok, member_reloaded} = Ash.get(Mv.Membership.Member, member.id, actor: actor)
+      assert member_reloaded != nil
+
+      # MemberGroup should be deleted
+      {:ok, mgs} =
+        Ash.read(
+          Mv.Membership.MemberGroup
+          |> Ash.Query.filter(expr(group_id == ^group.id)),
+          actor: actor,
+          domain: Mv.Membership
+        )
+
+      assert mgs == []
+    end
+  end
+end
diff --git a/test/membership/member_groups_relationship_test.exs b/test/membership/member_groups_relationship_test.exs
new file mode 100644
index 0000000..a72c8bc
--- /dev/null
+++ b/test/membership/member_groups_relationship_test.exs
@@ -0,0 +1,197 @@
+defmodule Mv.Membership.MemberGroupsRelationshipTest do
+  @moduledoc """
+  Tests for Member resource extension with groups relationship.
+  """
+  use Mv.DataCase, async: true
+
+  alias Mv.Membership
+
+  require Ash.Query
+  import Ash.Expr
+
+  setup do
+    system_actor = Mv.Helpers.SystemActor.get_system_actor()
+    %{actor: system_actor}
+  end
+
+  describe "Relationships" do
+    test "member has many_to_many groups relationship (load with preloading)", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group1} = Membership.create_group(%{name: "Group One"}, actor: actor)
+      {:ok, group2} = Membership.create_group(%{name: "Group Two"}, actor: actor)
+
+      {:ok, _mg1} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group1.id},
+          actor: actor
+        )
+
+      {:ok, _mg2} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group2.id},
+          actor: actor
+        )
+
+      # Load member with groups
+      {:ok, member_with_groups} =
+        Ash.load(member, :groups, actor: actor, domain: Mv.Membership)
+
+      assert length(member_with_groups.groups) == 2
+      assert Enum.any?(member_with_groups.groups, &(&1.id == group1.id))
+      assert Enum.any?(member_with_groups.groups, &(&1.id == group2.id))
+    end
+
+    test "load multiple members with groups preloaded (N+1 prevention)", %{actor: actor} do
+      {:ok, member1} = Membership.create_member(%{email: "member1@test.com"}, actor: actor)
+      {:ok, member2} = Membership.create_member(%{email: "member2@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      {:ok, _mg1} =
+        Membership.create_member_group(%{member_id: member1.id, group_id: group.id},
+          actor: actor
+        )
+
+      {:ok, _mg2} =
+        Membership.create_member_group(%{member_id: member2.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Load all members with groups in single query
+      {:ok, members} =
+        Ash.read(Mv.Membership.Member, actor: actor, domain: Mv.Membership, load: [:groups])
+
+      member1_loaded = Enum.find(members, &(&1.id == member1.id))
+      member2_loaded = Enum.find(members, &(&1.id == member2.id))
+
+      assert length(member1_loaded.groups) == 1
+      assert length(member2_loaded.groups) == 1
+      assert hd(member1_loaded.groups).id == group.id
+      assert hd(member2_loaded.groups).id == group.id
+    end
+  end
+
+  describe "Member-Group Association Operations" do
+    test "add member to group via Ash API", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      assert {:ok, member_group} =
+               Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+                 actor: actor
+               )
+
+      assert member_group.member_id == member.id
+      assert member_group.group_id == group.id
+    end
+
+    test "remove member from group via Ash API", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      {:ok, member_group} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Remove association
+      :ok = Membership.destroy_member_group(member_group, actor: actor)
+
+      # Verify association is removed
+      {:ok, mgs} =
+        Ash.read(
+          Mv.Membership.MemberGroup
+          |> Ash.Query.filter(expr(member_id == ^member.id and group_id == ^group.id)),
+          actor: actor,
+          domain: Mv.Membership
+        )
+
+      assert mgs == []
+    end
+
+    test "add member to multiple groups in single operation", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group1} = Membership.create_group(%{name: "Group One"}, actor: actor)
+      {:ok, group2} = Membership.create_group(%{name: "Group Two"}, actor: actor)
+      {:ok, group3} = Membership.create_group(%{name: "Group Three"}, actor: actor)
+
+      # Add to all groups
+      {:ok, _mg1} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group1.id},
+          actor: actor
+        )
+
+      {:ok, _mg2} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group2.id},
+          actor: actor
+        )
+
+      {:ok, _mg3} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group3.id},
+          actor: actor
+        )
+
+      # Verify all associations exist
+      {:ok, member_with_groups} =
+        Ash.load(member, :groups, actor: actor, domain: Mv.Membership)
+
+      assert length(member_with_groups.groups) == 3
+    end
+  end
+
+  describe "Edge Cases" do
+    test "adding member to same group twice fails (duplicate prevention)", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      {:ok, _mg1} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+          actor: actor
+        )
+
+      # Try to add again
+      assert {:error, %Ash.Error.Invalid{}} =
+               Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+                 actor: actor
+               )
+    end
+
+    test "removing member from group they're not in (idempotent, no error)", %{actor: actor} do
+      {:ok, member} = Membership.create_member(%{email: "test@test.com"}, actor: actor)
+      {:ok, group} = Membership.create_group(%{name: "Test Group"}, actor: actor)
+
+      # Verify no association exists
+      {:ok, nil} =
+        Ash.read_one(
+          Mv.Membership.MemberGroup
+          |> Ash.Query.filter(expr(member_id == ^member.id and group_id == ^group.id)),
+          actor: actor,
+          domain: Mv.Membership
+        )
+
+      # Test idempotency: Create association, delete it, then try to delete again
+      # This verifies that destroy_member_group is idempotent
+      {:ok, member_group} =
+        Membership.create_member_group(%{member_id: member.id, group_id: group.id},
+          actor: actor
+        )
+
+      # First deletion should succeed
+      assert :ok = Membership.destroy_member_group(member_group, actor: actor)
+
+      # Verify association is deleted
+      {:ok, nil} =
+        Ash.read_one(
+          Mv.Membership.MemberGroup
+          |> Ash.Query.filter(expr(id == ^member_group.id)),
+          actor: actor,
+          domain: Mv.Membership
+        )
+
+      # Try to destroy again - should be idempotent (either succeed or return not found error)
+      # Note: This tests the idempotency of the destroy action
+      result = Membership.destroy_member_group(member_group, actor: actor)
+
+      # Should either succeed (idempotent) or return an error (not found)
+      # Both behaviors are acceptable for idempotency
+      assert result == :ok || match?({:error, _}, result)
+    end
+  end
+end