improvement: add postgres_reference_expr callback (#438)

Custom types can use this to transform bare attribute expressions into
boolean expressions for filters.

lib/sql_implementation.ex

@@ -153,6 +153,33 @@ defmodule AshPostgres.SqlImplementation do
     {:ok, Ecto.Query.dynamic(fragment("(? <=> ?)", ^arg1, ^arg2)), acc}
   end
 
+  def expr(
+    query,
+    %Ash.Query.Ref{
+      attribute: %Ash.Resource.Attribute{
+        type: attr_type,
+        constraints: constraints,
+      },
+      bare?: true
+    } = ref,
+    bindings,
+    embedded?,
+    acc,
+    type
+  ) do
+    if function_exported?(attr_type, :postgres_reference_expr, 3) do
+      non_bare_ref = %Ash.Query.Ref{ ref | bare?: nil }
+      {expr, acc} = AshSql.Expr.dynamic_expr(query, non_bare_ref, bindings, embedded?, type, acc)
+
+      case attr_type.postgres_reference_expr(attr_type, constraints, expr) do
+        {:ok, bare_expr} -> {:ok, bare_expr, acc}
+        :error -> :error
+      end
+    else
+      :error
+    end
+  end
+
   def expr(
         _query,
         _expr,

lib/type.ex

@@ -8,12 +8,21 @@ defmodule AshPostgres.Type do
   @callback value_to_postgres_default(Ash.Type.t(), Ash.Type.constraints(), term) ::
               {:ok, String.t()} | :error
 
+  @callback postgres_reference_expr(Ash.Type.t(), Ash.Type.constraints(), term) ::
+              {:ok, term} | :error
+
+  @optional_callbacks value_to_postgres_default: 3,
+                      postgres_reference_expr: 3
+
   defmacro __using__(_) do
     quote do
       @behaviour AshPostgres.Type
+
       def value_to_postgres_default(_, _, _), do: :error
+      def postgres_reference_expr(_, _, _), do: :error
 
-      defoverridable value_to_postgres_default: 3
+      defoverridable value_to_postgres_default: 3,
+                     postgres_reference_expr: 3
     end
   end
 end