feat: custom field deletion

lib/membership/custom_field.ex

@@ -28,7 +28,10 @@ defmodule Mv.Membership.CustomField do
   ## Constraints
   - Name must be unique across all custom fields
   - Name maximum length: 100 characters
-  - Cannot delete a custom field that has existing custom field values (RESTRICT)
+  - Deleting a custom field will cascade delete all associated custom field values
+
+  ## Calculations
+  - `assigned_members_count` - Returns the number of distinct members with values for this custom field
 
   ## Examples
       # Create a new custom field
@@ -55,7 +58,7 @@ defmodule Mv.Membership.CustomField do
   end
 
   actions do
-    defaults [:read, :update, :destroy]
+    defaults [:read, :update]
     default_accept [:name, :value_type, :description, :immutable, :required]
 
     create :create do
@@ -63,6 +66,17 @@ defmodule Mv.Membership.CustomField do
       change Mv.Membership.CustomField.Changes.GenerateSlug
       validate string_length(:slug, min: 1)
     end
+
+    destroy :destroy_with_values do
+      primary? true
+    end
+
+    read :prepare_deletion do
+      argument :id, :uuid, allow_nil?: false
+
+      filter expr(id == ^arg(:id))
+      prepare build(load: [:assigned_members_count])
+    end
   end
 
   attributes do
@@ -111,6 +125,17 @@ defmodule Mv.Membership.CustomField do
     has_many :custom_field_values, Mv.Membership.CustomFieldValue
   end
 
+  calculations do
+    calculate :assigned_members_count,
+              :integer,
+              expr(
+                fragment(
+                  "(SELECT COUNT(DISTINCT member_id) FROM custom_field_values WHERE custom_field_id = ?)",
+                  id
+                )
+              )
+  end
+
   identities do
     identity :unique_name, [:name]
     identity :unique_slug, [:slug]