Repo.aggregate: wrap query into a subquery if it has combinations

lib/ecto/repo.ex

@@ -750,7 +750,7 @@ defmodule Ecto.Repo do
   @doc """
   Calculate the given `aggregate`.
 
-  If the query has a limit, offset or distinct set, it will be
+  If the query has a limit, offset, distinct or combination set, it will be
   automatically wrapped in a subquery in order to return the
   proper result.
 

lib/ecto/repo/queryable.ex

@@ -473,7 +473,7 @@ defmodule Ecto.Repo.Queryable do
   defp query_for_aggregate(queryable, aggregate) do
     query =
       case prepare_for_aggregate(queryable) do
-        %{distinct: nil, limit: nil, offset: nil} = query ->
+        %{distinct: nil, limit: nil, offset: nil, combinations: []} = query ->
           %{query | order_bys: []}
 
         query ->
@@ -491,7 +491,7 @@ defmodule Ecto.Repo.Queryable do
 
     query =
       case prepare_for_aggregate(queryable) do
-        %{distinct: nil, limit: nil, offset: nil} = query ->
+        %{distinct: nil, limit: nil, offset: nil, combinations: []} = query ->
           %{query | order_bys: []}
 
         query ->

test/ecto/repo_test.exs

@@ -355,7 +355,7 @@ defmodule Ecto.RepoTest do
       assert inspect(query) == "#Ecto.Query<from m0 in Ecto.RepoTest.MySchemaWithAssoc, select: count(m0.id)>"
     end
 
-    test "removes order by from query without distinct/limit/offset" do
+    test "removes order by from query without distinct/limit/offset/combinations" do
       from(MySchema, order_by: :id) |> TestRepo.aggregate(:count, :id)
       assert_received {:all, query}
       assert inspect(query) == "#Ecto.Query<from m0 in Ecto.RepoTest.MySchema, select: count(m0.id)>"
@@ -376,6 +376,17 @@ defmodule Ecto.RepoTest do
                 "  select: %{id: m0.id}), select: count(m0.id)>"
     end
 
+    test "uses subqueries with combinations" do
+      from(MySchema, union: ^from(MySchema)) |> TestRepo.aggregate(:count, :id)
+      assert_received {:all, query}
+
+      assert inspect(query) ==
+               "#Ecto.Query<from m0 in subquery(from m0 in Ecto.RepoTest.MySchema,\n" <>
+                 "  union: (from m0 in Ecto.RepoTest.MySchema,\n" <>
+                 "  select: m0),\n" <>
+                 "  select: %{id: m0.id}), select: count(m0.id)>"
+    end
+
     test "raises when aggregating with group_by" do
       assert_raise Ecto.QueryError, ~r"cannot aggregate on query with group_by", fn ->
         from(MySchema, group_by: [:id]) |> TestRepo.aggregate(:count, :id)