Fix empty array compare in PostgreSQL (#452)

Author: Zarathustra2

integration_test/sql/sql.exs

@@ -6,6 +6,7 @@ defmodule Ecto.Integration.SQLTest do
   alias Ecto.Integration.Barebone
   alias Ecto.Integration.Post
   alias Ecto.Integration.CorruptedPk
+  alias Ecto.Integration.Tag
   import Ecto.Query, only: [from: 2]
 
   test "fragmented types" do
@@ -28,6 +29,18 @@ defmodule Ecto.Integration.SQLTest do
     assert result.rows == [[[text1, text2]]]
   end
 
+  @tag :array_type
+  test "Converts empty array correctly" do
+    result = TestRepo.query!("SELECT array[1,2,3] = $1", [[]])
+    assert result.rows == [[false]]
+
+    result = TestRepo.query!("SELECT array[]::integer[] = $1", [[]])
+    assert result.rows == [[true]]
+
+    query = from t in Tag, where: t.items == []
+    assert [] == TestRepo.all(query)
+  end
+
   test "query!/4 with dynamic repo" do
     TestRepo.put_dynamic_repo(:unknown)
     assert_raise RuntimeError, ~r/:unknown/, fn -> TestRepo.query!("SELECT 1") end

lib/ecto/adapters/postgres/connection.ex

@@ -746,6 +746,20 @@ if Code.ensure_loaded?(Postgrex) do
       end
     end
 
+    defp expr([], _sources, _query) do
+      # We cannot compare in postgres with the empty array
+      # i. e. `where array_column = ARRAY[];`
+      # as that will result in an error:
+      #   ERROR:  cannot determine type of empty array
+      #   HINT:  Explicitly cast to the desired type, for example ARRAY[]::integer[].
+      #
+      # On the other side comparing with '{}' works
+      # because '{}' represents the pseudo-type "unknown"
+      # and thus the type gets inferred based on the column
+      # it is being compared to so `where array_column = '{}';` works.
+      "'{}'"
+    end
+
     defp expr(list, sources, query) when is_list(list) do
       ["ARRAY[", intersperse_map(list, ?,, &expr(&1, sources, query)), ?]]
     end

test/ecto/adapters/postgres_test.exs

@@ -752,6 +752,9 @@ defmodule Ecto.Adapters.PostgresTest do
 
     query = Schema |> select([], fragment("?", ~w(abc def))) |> plan()
     assert all(query) == ~s{SELECT ARRAY['abc','def'] FROM "schema" AS s0}
+
+    query = Schema |> where([s], s.w == []) |> select([s], s.w) |> plan()
+    assert all(query) == ~s{SELECT s0."w" FROM "schema" AS s0 WHERE (s0."w" = '\{\}')}
   end
 
   test "interpolated values" do