Add Rubocop overfetch cop

lib/rubocop/cop/graphql/overfetch.rb

@@ -0,0 +1,63 @@
+require "active_support/inflector"
+require "graphql"
+require "graphql/client/erubis"
+require "rubocop"
+
+module RuboCop
+  module Cop
+    module GraphQL
+      # Public: Rubocop for catching overfetched fields in ERB templates.
+      class Overfetch < Cop
+        def_node_search :send_methods, "(send ...)"
+
+        def investigate(processed_source)
+          erb = File.read(processed_source.buffer.name)
+          query, = ::GraphQL::Client::Erubis.extract_graphql_section(erb)
+          return unless query
+
+          aliases = {}
+          fields = {}
+          ranges = {}
+
+          # TODO: Use GraphQL client parser
+          document = ::GraphQL.parse(query.gsub(/::/, "__"))
+
+          visitor = ::GraphQL::Language::Visitor.new(document)
+          visitor[::GraphQL::Language::Nodes::Field] << ->(node, _parent) do
+            name = node.alias || node.name
+            fields[name] ||= 0
+            field_aliases(name).each { |n| (aliases[n] ||= []) << name }
+            ranges[name] ||= source_range(processed_source.buffer, node.line, 0)
+          end
+          visitor.visit
+
+          send_methods(processed_source.ast).each do |node|
+            _receiver, method_name, *_args = *node
+            aliases.fetch(method_name.to_s, []).each do |field_name|
+              fields[field_name] += 1
+            end
+          end
+
+          fields.each do |field, count|
+            next if count > 0
+            add_offense(nil, ranges[field], "GraphQL field '#{field}' query but was not used in template.")
+          end
+        end
+
+        def field_aliases(name)
+          names = Set.new
+
+          names << name
+          names << "#{name}?"
+
+          names << underscore_name = ActiveSupport::Inflector.underscore(name)
+          names << "#{underscore_name}?"
+
+          names << "each_node" if name == "edges" || name == "node"
+
+          names
+        end
+      end
+    end
+  end
+end

test/test_rubocop_overfetch.rb

@@ -0,0 +1,35 @@
+require "graphql/client/erubis"
+require "rubocop/cop/graphql/overfetch"
+require "minitest/autorun"
+
+class TestRubocopOverfetch < MiniTest::Test
+  Root = File.expand_path("..", __FILE__)
+
+  def setup
+    config = RuboCop::Config.new
+    @cop = RuboCop::Cop::GraphQL::Overfetch.new(config)
+  end
+
+  def test_all_fields_used
+    investigate(@cop, "#{Root}/views/users/show.html.erb")
+
+    assert_empty @cop.offenses.map(&:message)
+  end
+
+  def test_field_unused
+    investigate(@cop, "#{Root}/views/users/overfetch.html.erb")
+
+    assert_equal 1, @cop.offenses.count
+    assert_equal "GraphQL field 'birthday' query but was not used in template.", @cop.offenses.first.message
+  end
+
+  private
+
+  def investigate(cop, path)
+    engine = GraphQL::Client::Erubis.new(File.read(path))
+    processed_source = RuboCop::ProcessedSource.new(engine.src, RUBY_VERSION.to_f, path)
+    commissioner = RuboCop::Cop::Commissioner.new([cop], [], raise_error: true)
+    commissioner.investigate(processed_source)
+    commissioner
+  end
+end

test/views/users/overfetch.html.erb

@@ -0,0 +1,7 @@
+<%graphql
+  fragment User on User {
+    login
+    birthday
+  }
+%>
+<%= user.login %>