Fix generated foreign key constraint on specially named references

lib/data_layer.ex

@@ -2682,8 +2682,15 @@ defmodule AshPostgres.DataLayer do
     resource
     |> Ash.Resource.Info.relationships()
     |> Enum.reduce(changeset, fn relationship, changeset ->
+      # Check if there's a custom reference name defined in the DSL
       name =
-        "#{AshPostgres.DataLayer.Info.table(resource)}_#{relationship.source_attribute}_fkey"
+        case AshPostgres.DataLayer.Info.reference(resource, relationship.name) do
+          %{name: custom_name} when not is_nil(custom_name) ->
+            custom_name
+
+          _ ->
+            "#{AshPostgres.DataLayer.Info.table(resource)}_#{relationship.source_attribute}_fkey"
+        end
 
       case repo.default_constraint_match_type(:foreign, name) do
         {:regex, regex} ->

test/references_test.exs

@@ -105,4 +105,27 @@
       end
     end
   end
+
+  test "named reference results in properly applied foreign_key_constraint/3 on the underlying changeset" do
+    # Create a comment with an invalid post_id
+    assert {:error, %Ash.Error.Invalid{errors: errors}} =
+             AshPostgres.Test.Comment
+             |> Ash.Changeset.for_create(:create, %{
+               title: "Test Comment",
+               # This post doesn't exist
+               post_id: Ash.UUID.generate()
+             })
+             |> Ash.create()
+
+    assert [
+             %Ash.Error.Changes.InvalidAttribute{
+               field: :post_id,
+               message: "does not exist",
+               private_vars: private_vars
+             }
+           ] = errors
+
+    assert Keyword.get(private_vars, :constraint) == "special_name_fkey"
+    assert Keyword.get(private_vars, :constraint_type) == :foreign_key
+  end
 end