Call prepare on lists of input objects

When the argument is a list of input objects we need to call prepare on
each item of the list

lib/graphql/schema/argument.rb

@@ -208,6 +208,8 @@ def statically_coercible?
       def prepare_value(obj, value, context: nil)
         if value.is_a?(GraphQL::Schema::InputObject)
           value = value.prepare
+        elsif type.list? && value[0].is_a?(GraphQL::Schema::InputObject)
+          value = value.map(&:prepare)
         end
 
         Schema::Validator.validate!(validators, obj, context, value)

spec/graphql/schema/input_object_spec.rb

@@ -407,6 +407,14 @@ def inputs(input:)
         def prepare_once(input:)
           input.prepared_count
         end
+
+        field :prepare_list, [Int] do
+          argument :input, [OnlyOnePrepareInputObject]
+        end
+
+        def prepare_list(input:)
+          input.map(&:prepared_count)
+        end
       end
 
       class Schema < GraphQL::Schema
@@ -428,6 +436,11 @@ class Schema < GraphQL::Schema
       assert_equal 1, res["data"]["prepareOnce"]
     end
 
+    it "calls prepare on lists of input objects" do
+      res = InputObjectPrepareObjectTest::Schema.execute("{ prepareList( input:[{ i: 1 }, { i: 1}]) }")
+      assert_equal [1, 1], res["data"]["prepareList"]
+    end
+
     it "calls prepare on the input object (variable)" do
       query_str = <<-GRAPHQL
       query ($input: RangeInput!){ inputs(input: $input) }