Merge pull request #1758 from rmosolgo/field-filter-instances

Field Extensions (second try)

Author: Robert Mosolgo

guides/type_definitions/field_extensions.md

@@ -0,0 +1,104 @@
+---
+layout: guide
+doc_stub: false
+search: true
+section: Type Definitions
+title: Field Extensions
+desc: Programmatically modify field configuration and resolution
+index: 10
+class_based_api: true
+---
+
+{{ "GraphQL::Schema::FieldExtension" | api_doc }} provides a way to modify user-defined fields in a programmatic way. For example, Relay connections are implemented as a field extension ({{ "GraphQL::Schema::Field::ConnectionExtension" | api_doc }}).
+
+### Making a new extension
+
+Field extensions are subclasses of {{ "GraphQL::Schema::FieldExtension" | api_doc }}:
+
+```ruby
+class MyExtension < GraphQL::Schema::FieldExtension
+end
+```
+
+### Using an extension
+
+Defined extensions can be added to fields using the `extensions: [...]` option or the `extension(...)` method:
+
+```ruby
+field :name, String, null: false, extensions: [UpcaseExtension]
+# or:
+field :description, String, null: false do
+  extension(UpcaseExtension)
+end
+```
+
+See below for how extensions may modify fields.
+
+### Modifying field configuration
+
+When extensions are attached, they are initialized with a `field:` and `options:`. Then, `#apply` is called, when they may extend the field they're attached to. For example:
+
+```ruby
+class SearchableExtension < GraphQL::Schema::FieldExtension
+  def apply
+    # add an argument to this field:
+    field.argument(:query, String, required: false, description: "A search query")
+  end
+end
+```
+
+This way, an extension can encapsulate a behavior requiring several configuration options.
+
+### Modifying field execution
+
+Extensions have two hooks that wrap field resolution. Since GraphQL-Ruby supports deferred execution, these hooks _might not_ be called back-to-back.
+
+First, {{ "GraphQL::Schema::FieldExtension#before_resolve" | api_doc }} is called. `before_resolve` should `yield(object, arguments)` to continue execution. If it doesn't `yield`, then the field won't resolve, and the methods return value will be returned to GraphQL instead.
+
+After resolution, {{ "GraphQL::Schema::FieldExtension#after_resolve" | api_doc }} is called. Whatever that method returns will be used as the field's return value.
+
+See the linked API docs for the parameters of those methods.
+
+#### Execution "memo"
+
+One parameter to `after_resolve` deserves special attention: `memo:`. `before_resolve` _may_ yield a third value. For example:
+
+```ruby
+def before_resolve(object:, arguments:, **rest)
+  # yield the current time as `memo`
+  yield(object, arguments, Time.now.to_i)
+end
+```
+
+If a third value is yielded, it will be passed to `after_resolve` as `memo:`, for example:
+
+```ruby
+def after_resolve(value:, memo:, **rest)
+  puts "Elapsed: #{Time.now.to_i - memo}"
+  # Return the original value
+  value
+end
+```
+
+This allows the `before_resolve` hook to pass data to `after_resolve`.
+
+Instance variables may not be used because, in a given GraphQL query, the same field may be resolved several times concurrently, and that would result in overriding the instance variable in an unpredictable way. (In fact, extensions are frozen to prevent instance variable writes.)
+
+### Extension options
+
+The `extension(...)` method takes an optional second argument, for example:
+
+```ruby
+extension(LimitExtension, limit: 20)
+```
+
+In this case, `{limit: 20}` will be passed as `options:` to `#initialize` and `options[:limit]` will be `20`.
+
+For example, options can be used for modifying execution:
+
+```ruby
+def after_resolve(value:, **rest)
+  # Apply the limit from the options
+  value.limit(options[:limit])
+end
+```

lib/graphql/relay/connection_instrumentation.rb

@@ -32,7 +32,9 @@ def self.default_arguments
       # - Merging in the default arguments
       # - Transforming its resolve function to return a connection object
       def self.instrument(type, field)
-        if field.connection?
+        # Don't apply the wrapper to class-based fields, since they
+        # use Schema::Field::ConnectionFilter
+        if field.connection? && !field.metadata[:type_class]
           connection_arguments = default_arguments.merge(field.arguments)
           original_resolve = field.resolve_proc
           original_lazy_resolve = field.lazy_resolve_proc

lib/graphql/schema.rb

@@ -19,23 +19,24 @@
 require "graphql/schema/warden"
 require "graphql/schema/build_from_definition"
 
-
 require "graphql/schema/member"
 require "graphql/schema/wrapper"
 require "graphql/schema/list"
 require "graphql/schema/non_null"
 require "graphql/schema/argument"
 require "graphql/schema/enum_value"
 require "graphql/schema/enum"
+require "graphql/schema/field_extension"
 require "graphql/schema/field"
 require "graphql/schema/input_object"
 require "graphql/schema/interface"
+require "graphql/schema/scalar"
+require "graphql/schema/object"
+require "graphql/schema/union"
+
 require "graphql/schema/resolver"
 require "graphql/schema/mutation"
 require "graphql/schema/relay_classic_mutation"
-require "graphql/schema/object"
-require "graphql/schema/scalar"
-require "graphql/schema/union"
 
 module GraphQL
   # A GraphQL schema which may be queried with {GraphQL::Query}.