Merge pull request #4186 from rmosolgo/merge-into-dataloader

Add .merge for pre-filling dataloader caches

guides/dataloader/sources.md

@@ -135,3 +135,16 @@ end
 For a more robust asynchronous task primitive, check out [`Concurrent::Future`](http://ruby-concurrency.github.io/concurrent-ruby/master/Concurrent/Future.html).
 
 Ruby 3.0 added built-in support for yielding Fibers that make I/O calls -- hopefully a future GraphQL-Ruby version will work with that!
+
+## Filling the Dataloader Cache
+
+If you load records from the database, you can use them to populate a source's cache by using {{ "Dataloader::Source#merge" | api_doc }}. For example:
+
+```ruby
+# Build a `{ key => value }` map to populate the cache
+comments_by_id = post.comments.each_with_object({}) { |comment, hash| hash[comment.id] = comment }
+# Merge the map into the source's cache
+dataloader.with(Sources::ActiveRecordObject, Comment).merge(comments_by_id)
+```
+
+After that, any calls to `.load(id)` will use those already-loaded records if they're available.

lib/graphql/dataloader/source.rb

@@ -86,6 +86,15 @@ def pending?
         !@pending_keys.empty?
       end
 
+      # Add these key-value pairs to this source's cache
+      # (future loads will use these merged values).
+      # @param results [Hash<Object => Object>] key-value pairs to cache in this source
+      # @return [void]
+      def merge(results)
+        @results.merge!(results)
+        nil
+      end
+
       # Called by {GraphQL::Dataloader} to resolve and pending requests to this source.
       # @api private
       # @return [void]

spec/graphql/dataloader_spec.rb

@@ -718,7 +718,6 @@ def self.included(child_class)
           assert_equal expected_log, database_log
         end
 
-
         it "supports general usage" do
           a = b = c = nil
 
@@ -773,6 +772,21 @@ def self.included(child_class)
           ]
           assert_equal expected_log, database_log
         end
+
+        it "uses cached values from .merge" do
+          query_str = "{ ingredient(id: 1) { id name } }"
+          assert_equal "Wheat", schema.execute(query_str)["data"]["ingredient"]["name"]
+          assert_equal [[:mget, ["1"]]], database_log
+          database_log.clear
+
+          dataloader = schema.dataloader_class.new
+          data_source = dataloader.with(FiberSchema::DataObject)
+          data_source.merge({ "1" => { name: "Kamut", id: "1", type: "Grain" } })
+          assert_equal "Kamut", data_source.load("1")[:name]
+          res = schema.execute(query_str, context: { dataloader: dataloader })
+          assert_equal [], database_log
+          assert_equal "Kamut", res["data"]["ingredient"]["name"]
+        end
       end
     end
   end