Prevent loader re-use after mutations

[dylanahsmith]

cc @staugaard

Problem

The loader re-use added by #53 also re-used the loaders and their resolved promises after a mutation field is resolved.

GraphQL::GraphQLTest#test_mutation_execution was a test that was supposed to catch this bug, but it was using CounterLoader.for(ctx).load where load was defined in CounterLoader with def load(key=Object.new), so it was actually using a unique key for each load which prevented it from ever re-using the result of a previous load.

Solution

I've reverted #53 to fix the bug and updated the test to use override cache_key to use the object_id of the array that is used to hold the counter.

If the test is run without the revert, then it has the expected failures of

  1) Failure:
GraphQL::GraphQLTest#test_mutation_execution [test/graphql_test.rb:315]:
--- expected
+++ actual
@@ -1 +1 @@
-{"data"=>{"count1"=>0, "incr1"=>{"value"=>1, "load_value"=>1}, "count2"=>1, "incr2"=>{"value"=>2, "load_value"=>2}}}
+{"data"=>{"count1"=>0, "incr1"=>{"value"=>0, "load_value"=>0}, "count2"=>0, "incr2"=>{"value"=>0, "load_value"=>0}}}

[dylanahsmith]

I will need to reopen #43 after this

[staugaard]

Maybe we could clear the loader cache after executing a mutation