Don't bomb if the query passed into an ecto association already includes a select value.

lib/graphql/relay/connection/ecto.ex

@@ -30,7 +30,7 @@ if Code.ensure_loaded?(Ecto) do
         nil -> false
         _ ->
           first_limit = first + 1
-          has_more_records_query = from things in query, limit: ^first_limit
+          has_more_records_query = remove_select(from things in query, limit: ^first_limit)
           has_more_records_query = from things in has_more_records_query, select: count(things.id)
           repo.one(has_more_records_query) > first
       end
@@ -39,7 +39,7 @@ if Code.ensure_loaded?(Ecto) do
         nil -> false
         _ ->
           last_limit = last + 1
-          has_prev_records_query = from things in query, limit: ^last_limit
+          has_prev_records_query = remove_select(from things in query, limit: ^last_limit)
           has_prev_records_query = from things in has_prev_records_query, select: count(things.id)
           repo.one(has_prev_records_query) > last
       end
@@ -114,8 +114,15 @@ if Code.ensure_loaded?(Ecto) do
     end
 
     def connection_count(repo, query) do
+      query = remove_select(query)
       count_query = from things in query, select: count(things.id)
       repo.one(count_query)
     end
+
+    # Remove select if it exists so that we avoid `only one select
+    # expression is allowed in query` Ecto exception
+    defp remove_select(query) do
+      %{ query | select: nil }
+    end
   end
 end

test/graphql/relay/connection/ecto_test.exs

@@ -59,6 +59,12 @@ defmodule GraphQL.Relay.Connection.EctoTest do
     Enum.at(letters, 4)
   end
 
+  test "querying for counts does not raise exception if select already exists" do
+    query = letters_query
+      |> select([l], %{id: l.id, letter: l.letter})
+    assert(Connection.Ecto.resolve(query, %{repo: Repo}))
+  end
+
   test "basic slicing: returns all elements without filters" do
     expected = %{
       edges: [