test: add failing test for cond/if calculation in query filters (#766)

cond/if expressions used in query filters produce a
DBConnection.EncodeError because the THEN clause literals in the
generated CASE WHEN are not type-cast (e.g. THEN 3) when the
expression appears in a WHERE clause, while they ARE properly cast
(e.g. THEN 3::bigint) in a SELECT clause.

The root cause is in ash_sql's expr.ex: when embedded? is true
(which it is for cond/if expression nodes), the literal handler
at default_dynamic_expr/6 skips type casting entirely, so the
Fragment parameters end up as {value, :any}. Postgrex then cannot
infer the correct encoding from context and defaults to text,
causing the binary vs integer mismatch.

Author: diogomrts

test/calculation_test.exs

@@ -1100,6 +1100,31 @@ defmodule AshPostgres.CalculationTest do
     assert full_name == "name"
   end
 
+  test "cond/if calculation can be used in query filters" do
+    post =
+      Post
+      |> Ash.Changeset.for_create(:create, %{title: "match", score: 75})
+      |> Ash.create!()
+
+    # Loading the cond-based calculation works fine
+    post = Ash.load!(post, :score_category)
+    assert post.score_category == 2
+
+    # Filtering by a cond-based calculation should also work.
+    # This currently fails with DBConnection.EncodeError because the THEN
+    # clause literals in the generated CASE WHEN are not type-cast when the
+    # expression appears in a WHERE clause (they ARE cast in SELECT).
+    assert [_] =
+             Post
+             |> Ash.Query.filter(score_category == 2)
+             |> Ash.read!()
+
+    assert [] =
+             Post
+             |> Ash.Query.filter(score_category == 3)
+             |> Ash.read!()
+  end
+
   test "calculation with fragment and cond returning integer doesn't cause Postgrex encoding error" do
     Post
     |> Ash.Changeset.for_create(:create, %{title: "hello ash lovers"})

test/support/resources/post.ex

@@ -1124,6 +1124,20 @@ defmodule AshPostgres.Test.Post do
 
     calculate(:c_times_p, :integer, expr(count_of_comments * count_of_linked_posts))
 
+
+    calculate(
+      :score_category,
+      :integer,
+      expr(
+        cond do
+          score > 100 -> 3
+          score > 50 -> 2
+          score > 0 -> 1
+          true -> 0
+        end
+      )
+    )
+
     calculate(
       :literal_map_in_expr,
       :map,