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