fix: raise error when reference is configured for non-belongs_to relationship

lib/verifiers/validate_references.ex

@@ -11,13 +11,29 @@ defmodule AshPostgres.Verifiers.ValidateReferences do
     dsl
     |> AshPostgres.DataLayer.Info.references()
     |> Enum.each(fn reference ->
-      if !Ash.Resource.Info.relationship(dsl, reference.relationship) do
-        raise Spark.Error.DslError,
-          path: [:postgres, :references, reference.relationship],
-          module: Verifier.get_persisted(dsl, :module),
-          message:
-            "Found reference configuration for relationship `#{reference.relationship}`, but no such relationship exists",
-          location: Spark.Dsl.Transformer.get_section_anno(dsl, [:postgres, :references])
+      relationship = Ash.Resource.Info.relationship(dsl, reference.relationship)
+
+      cond do
+        is_nil(relationship) ->
+          raise Spark.Error.DslError,
+            path: [:postgres, :references, reference.relationship],
+            module: Verifier.get_persisted(dsl, :module),
+            message:
+              "Found reference configuration for relationship `#{reference.relationship}`, but no such relationship exists",
+            location: Spark.Dsl.Transformer.get_section_anno(dsl, [:postgres, :references])
+
+        relationship.type != :belongs_to ->
+          raise Spark.Error.DslError,
+            path: [:postgres, :references, reference.relationship],
+            module: Verifier.get_persisted(dsl, :module),
+            message:
+              "Found reference configuration for relationship `#{reference.relationship}`, but it is a `#{relationship.type}` relationship. " <>
+                "References can only be configured for `belongs_to` relationships, because the foreign key is defined on the table with the `belongs_to` relationship. " <>
+                "To configure the behavior of this foreign key, add the reference configuration to the resource with the corresponding `belongs_to` relationship.",
+            location: Spark.Dsl.Transformer.get_section_anno(dsl, [:postgres, :references])
+
+        true ->
+          :ok
       end
     end)
 

test/references_test.exs

@@ -114,6 +114,68 @@ defmodule AshPostgres.ReferencesTest do
     assert io =~ "Unsupported match_type"
   end
 
+  test "raises error when reference is configured for a has_many relationship" do
+    Code.compiler_options(ignore_module_conflict: true)
+    on_exit(fn -> Code.compiler_options(ignore_module_conflict: false) end)
+
+    {_, io} =
+      with_io(:stderr, fn ->
+        defmodule Parent do
+          @moduledoc false
+          use Ash.Resource,
+            domain: nil,
+            data_layer: AshPostgres.DataLayer
+
+          attributes do
+            uuid_primary_key(:id)
+          end
+
+          relationships do
+            has_many :children, AshPostgres.ReferencesTest.Child
+          end
+
+          postgres do
+            table("parents")
+            repo(AshPostgres.TestRepo)
+
+            references do
+              reference :children, on_delete: :delete
+            end
+          end
+
+          actions do
+            defaults([:create, :read, :update, :destroy])
+          end
+        end
+
+        defmodule Child do
+          @moduledoc false
+          use Ash.Resource,
+            domain: nil,
+            data_layer: AshPostgres.DataLayer
+
+          attributes do
+            uuid_primary_key(:id)
+          end
+
+          relationships do
+            belongs_to :parent, AshPostgres.ReferencesTest.Parent
+          end
+
+          postgres do
+            table("children")
+            repo(AshPostgres.TestRepo)
+          end
+
+          actions do
+            defaults([:create, :read, :update, :destroy])
+          end
+        end
+      end)
+
+    assert io =~ "References can only be configured for `belongs_to` relationships"
+  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}} =