improvement: add binding() expression

lib/data_layer.ex

@@ -785,7 +785,8 @@ defmodule AshPostgres.DataLayer do
 
     functions = [
       AshPostgres.Functions.Like,
-      AshPostgres.Functions.ILike
+      AshPostgres.Functions.ILike,
+      AshPostgres.Functions.Binding
     ]
 
     functions =

lib/functions/binding.ex

@@ -0,0 +1,9 @@
+defmodule AshPostgres.Functions.Binding do
+  @moduledoc """
+  Refers to the current table binding.
+  """
+
+  use Ash.Query.Function, name: :binding
+
+  def args, do: [[]]
+end

lib/sql_implementation.ex

@@ -41,6 +41,21 @@ defmodule AshPostgres.SqlImplementation do
     {:ok, Ecto.Query.dynamic(fragment("'[]'::jsonb")), acc}
   end
 
+  def expr(query, %AshPostgres.Functions.Binding{}, _bindings, _embedded?, acc, _type) do
+    binding = AshSql.Bindings.get_binding(
+      query.__ash_bindings__.resource,
+      [],
+      query,
+      [:left, :inner, :root]
+    )
+
+    if is_nil(binding) do
+        raise "Error while constructing explicit `binding()` reference."
+    end
+
+    {:ok, Ecto.Query.dynamic([{^binding, row}], row), acc}
+  end
+
   def expr(
         query,
         %like{arguments: [arg1, arg2], embedded?: pred_embedded?},

test/calculation_test.exs

@@ -800,6 +800,20 @@ defmodule AshPostgres.CalculationTest do
              |> Ash.read_one!()
   end
 
+  test "binding() can be used to refer to the current binding in a fragment" do
+    post =
+      Post
+      |> Ash.Changeset.for_create(:create, %{})
+      |> Ash.create!()
+
+    post_id = post.id
+
+    assert [%{id: ^post_id}] =
+             Post
+             |> Ash.Query.filter(fragment("(?).id", binding()) == type(^post.id, :uuid))
+             |> Ash.read!()
+  end
+
   test "exists with a relationship that has a filtered read action works" do
     post =
       Post