Add Active Record AssociationLoader and RecordLoader examples (#64)

README.md

@@ -72,6 +72,12 @@ The loader class can be used from the resolve proc for a graphql field by callin
 resolve -> (obj, args, context) { RecordLoader.for(Product).load(args["id"]) }
 ```
 
+Although this library doesn't have a dependency on active record,
+the [examples directory](examples) has record and association loaders
+for active record which handles edge cases like type casting ids
+and overriding GraphQL::Batch::Loader#cache_key to load associations
+on records with the same id.
+
 ### Promises
 
 GraphQL::Batch::Loader#load returns a Promise using the [promise.rb gem](https://rubygems.org/gems/promise.rb) to provide a promise based API, so you can transform the query results using `.then`

examples/association_loader.rb

@@ -0,0 +1,48 @@
+class AssociationLoader < GraphQL::Batch::Loader
+  def self.validate(model, association_name)
+    new(model, association_name)
+    nil
+  end
+
+  def initialize(model, association_name)
+    @model = model
+    @association_name = association_name
+    validate
+  end
+
+  def load(record)
+    raise TypeError, "#{@model} loader can't load association for #{record.class}" unless record.is_a?(@model)
+    return Promise.resolve(read_association(record)) if association_loaded?(record)
+    super
+  end
+
+  # We want to load the associations on all records, even if they have the same id
+  def cache_key(record)
+    record.object_id
+  end
+
+  def perform(records)
+    preload_association(records)
+    records.each { |record| fulfill(record, read_association(record)) }
+  end
+
+  private
+
+  def validate
+    unless @model.reflect_on_association(@association_name)
+      raise ArgumentError, "No association #{@association_name} on #{@model}"
+    end
+  end
+
+  def preload_association(records)
+    ::ActiveRecord::Associations::Preloader.new.preload(records, @association_name)
+  end
+
+  def read_association(record)
+    record.public_send(@association_name)
+  end
+
+  def association_loaded?(record)
+    record.association(@association_name).loaded?
+  end
+end

examples/record_loader.rb

@@ -0,0 +1,28 @@
+class RecordLoader < GraphQL::Batch::Loader
+  def initialize(model, column: model.primary_key, where: nil)
+    @model = model
+    @column = column.to_s
+    @column_type = model.type_for_attribute(@column)
+    @where = where
+  end
+
+  def load(key)
+    super(@column_type.cast(key))
+  end
+
+  def perform(keys)
+    query(keys).each do |record|
+      value = @column_type.cast(record.public_send(@column))
+      fulfill(value, record)
+    end
+    keys.each { |key| fulfill(key, nil) unless fulfilled?(key) }
+  end
+
+  private
+
+  def query(keys)
+    scope = @model
+    scope = scope.where(@where) if @where
+    scope.where(@column => keys)
+  end
+end