Merge pull request #168 from Shopify/prime

Priming the cache

README.md

@@ -140,6 +140,22 @@ def product(id:)
 end
 ```
 
+### Priming the Cache
+
+You can prime the loader cache with a specific value, which can be useful in certain situations.
+
+```ruby
+def liked_products
+ liked_products = Product.where(liked: true).load
+ liked_products.each do |product|
+ RecordLoader.for(Product).prime(product.id, product)
+ end
+end
+```
+
+Priming will add key/value to the loader cache only if it didn't exist before.
+
+
 ## Unit Testing
 
 Your loaders can be tested outside of a GraphQL query by doing the

lib/graphql/batch/loader.rb

@@ -62,6 +62,10 @@ def load_many(keys)
  ::Promise.all(keys.map { |key| load(key) })
  end
 
+ def prime(key, value)
+ cache[cache_key(key)] ||= ::Promise.resolve(value).tap { |p| p.source = self }
+ end
+
  def resolve #:nodoc:
  return if resolved?
  load_keys = queue

test/loader_test.rb

@@ -215,4 +215,40 @@ def test_loader_with_failing_perform
  promise = ExplodingLoader.load([1]).then(nil, ->(err) { error_message = err.message } ).sync
  assert_equal 'perform failed', error_message
  end
+
+ def test_prime
+ loader = EchoLoader.for
+ loader.prime(:a, :prepared_value)
+
+ assert_equal :prepared_value, loader.load(:a).sync
+ assert_equal [:prepared_value, :b, :c], loader.load_many([:a, :b, :c]).sync
+ end
+
+ def test_will_not_call_perform_if_fully_primed
+ loader = ExplodingLoader.for
+ loader.prime(:a, 1)
+ loader.prime(:b, 2)
+
+ assert_equal [1, 2], loader.load_many([:a, :b]).sync
+ end
+
+ def test_priming_a_key_already_in_queue_does_nothing
+ loader = EchoLoader.for
+
+ promise = loader.load(:a)
+
+ loader.prime(:a, :not_a)
+
+ assert_equal :a, promise.sync
+ end
+
+ def test_prime_will_not_replace_already_cached_value
+ loader = EchoLoader.for
+
+ assert_equal :a, loader.load(:a).sync
+
+ loader.prime(:a, :not_a)
+
+ assert_equal :a, loader.load(:a).sync
+ end
 end