Don't accidentally lose includes in serialization

activemodel/CHANGELOG.md

+*   Don't only take the first hash key in serialization :includes. Doing so silently loses options in `includes: [:account, {address: {only: id}, user: {only: id}]`
+
+    *Mike Mangino*
 ## Rails 5.1.0.beta1 (February 23, 2017) ##
 
 *   Remove deprecated behavior that halts callbacks when the return is false.

activemodel/lib/active_model/serialization.rb

@@ -177,7 +177,7 @@ def serializable_add_includes(options = {}) #:nodoc:
         return unless includes = options[:include]
 
         unless includes.is_a?(Hash)
-          includes = Hash[Array(includes).map { |n| n.is_a?(Hash) ? n.to_a.first : [n, {}] }]
+          includes = Hash[Array(includes).flat_map { |n| n.is_a?(Hash) ? n.to_a : [[n, {}]] }]
         end
 
         includes.each do |association, opts|

activemodel/test/cases/serialization_test.rb

@@ -144,6 +144,12 @@ def test_include_with_options
     assert_equal expected, @user.serializable_hash(include: { address: { only: "street" } })
   end
 
+  def test_multiple_includes_with_options
+    expected = { "email" => "david@example.com", "gender" => "male", "name" => "David",
+                "address" => { "street" => "123 Lane" } }
+    assert_equal expected, @user.serializable_hash(include: { address: { only: "street" } })
+  end
+
   def test_nested_include
     @user.friends.first.friends = [@user]
     expected = { "email" => "david@example.com", "gender" => "male", "name" => "David",
@@ -168,8 +174,8 @@ def test_except_include
   def test_multiple_includes_with_options
     expected = { "email" => "david@example.com", "gender" => "male", "name" => "David",
                 "address" => { "street" => "123 Lane" },
-                "friends" => [{ "name" => "Joe", "email" => "joe@example.com", "gender" => "male" },
-                           { "name" => "Sue", "email" => "sue@example.com", "gender" => "female" }] }
-    assert_equal expected, @user.serializable_hash(include: [{ address: { only: "street" } }, :friends])
+                "friends" => [{ "name" => "Joe" },
+                           { "name" => "Sue" }] }
+    assert_equal expected, @user.serializable_hash(include: [ address: { only: "street" } , friends: {only: "name"}])
   end
 end