From ab97b2947b4a5a5fff5017694e6e3424b85f6031 Mon Sep 17 00:00:00 2001
From: Zach Daniel <zach@zachdaniel.dev>
Date: Mon, 15 Jul 2024 10:25:39 -0400
Subject: [PATCH] improvement: add `binding()` expression

---
 lib/data_layer.ex         |  3 ++-
 lib/functions/binding.ex  |  9 +++++++++
 lib/sql_implementation.ex | 15 +++++++++++++++
 test/calculation_test.exs | 14 ++++++++++++++
 4 files changed, 40 insertions(+), 1 deletion(-)
 create mode 100644 lib/functions/binding.ex

diff --git a/lib/data_layer.ex b/lib/data_layer.ex
index 0ed9f72..450acc8 100644
--- a/lib/data_layer.ex
+++ b/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 =
diff --git a/lib/functions/binding.ex b/lib/functions/binding.ex
new file mode 100644
index 0000000..92fc9d6
--- /dev/null
+++ b/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
diff --git a/lib/sql_implementation.ex b/lib/sql_implementation.ex
index aa0a7c7..90414cf 100644
--- a/lib/sql_implementation.ex
+++ b/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?},
diff --git a/test/calculation_test.exs b/test/calculation_test.exs
index 186cf11..e15dbbc 100644
--- a/test/calculation_test.exs
+++ b/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