feat: add groups resource #371

lib/membership/member_group.ex

@@ -0,0 +1,128 @@
+defmodule Mv.Membership.MemberGroup do
+  @moduledoc """
+  Ash resource representing the join table for the many-to-many relationship
+  between Members and Groups.
+
+  ## Overview
+  MemberGroup is a join table that links members to groups. It enables the
+  many-to-many relationship where:
+  - A member can belong to multiple groups
+  - A group can contain multiple members
+
+  ## Attributes
+  - `member_id` - Foreign key to Member (required)
+  - `group_id` - Foreign key to Group (required)
+
+  ## Relationships
+  - `belongs_to :member` - Relationship to Member
+  - `belongs_to :group` - Relationship to Group
+
+  ## Constraints
+  - Unique constraint on `(member_id, group_id)` - prevents duplicate memberships
+  - CASCADE delete: Removing member removes all group associations
+  - CASCADE delete: Removing group removes all member associations
+
+  ## Examples
+      # Add member to group
+      MemberGroup.create!(%{member_id: member.id, group_id: group.id})
+
+      # Remove member from group
+      member_group = MemberGroup.get_by_member_and_group!(member.id, group.id)
+      MemberGroup.destroy!(member_group)
+  """
+  use Ash.Resource,
+    domain: Mv.Membership,
+    data_layer: AshPostgres.DataLayer
+
+  require Ash.Query
+  import Ash.Expr
+
+  postgres do
+    table "member_groups"
+    repo Mv.Repo
+  end
+
+  actions do
+    defaults [:read, :destroy]
+
+    create :create do
+      accept [:member_id, :group_id]
+    end
+  end
+
+  validations do
+    validate present(:member_id)
+    validate present(:group_id)
+
+    # Prevent duplicate associations
+    validate fn changeset, _context ->
+      member_id = Ash.Changeset.get_attribute(changeset, :member_id)
+      group_id = Ash.Changeset.get_attribute(changeset, :group_id)
+      current_id = Ash.Changeset.get_attribute(changeset, :id)
+
+      if member_id && group_id do
+        check_duplicate_association(member_id, group_id, current_id)
+      else
+        :ok
+      end
+    end
+  end
+
+  attributes do
+    uuid_v7_primary_key :id
+
+    attribute :member_id, :uuid do
+      allow_nil? false
+    end
+
+    attribute :group_id, :uuid do
+      allow_nil? false
+    end
+
+    timestamps()
+  end
+
+  relationships do
+    belongs_to :member, Mv.Membership.Member do
+      allow_nil? false
+    end
+
+    belongs_to :group, Mv.Membership.Group do
+      allow_nil? false
+    end
+  end
+
+  identities do
+    identity :unique_member_group, [:member_id, :group_id]
+  end
+
+  # Private helper function to check for duplicate associations
+  defp check_duplicate_association(member_id, group_id, exclude_id) do
+    alias Mv.Helpers
+    alias Mv.Helpers.SystemActor
+
+    query =
+      Mv.Membership.MemberGroup
+      |> Ash.Query.filter(member_id == ^member_id and group_id == ^group_id)
+      |> maybe_exclude_id(exclude_id)
+
+    system_actor = SystemActor.get_system_actor()
+    opts = Helpers.ash_actor_opts(system_actor)
+
+    case Ash.read(query, opts) do
+      {:ok, []} ->
+        :ok
+
+      {:ok, _} ->
+        {:error, field: :member_id, message: "Member is already in this group", value: member_id}
+
+      {:error, _reason} ->
+        # Fail-open: if query fails, allow operation to proceed
+        # Database constraint will catch duplicates anyway
+        :ok
+    end
+  end
+
+  defp maybe_exclude_id(query, nil), do: query
+  defp maybe_exclude_id(query, id), do: Ash.Query.filter(query, id != ^id)
+end