class GraphQL::StaticValidation::FieldsAreDefinedOnType

def validate(context)
  visitor = context.visitor
  visitor[GraphQL::Language::Nodes::Field] << -> (node, parent) {
    return if context.skip_field?(node.name)
    parent_type = context.object_types[-2]
    parent_type = parent_type.unwrap
    validate_field(context, node, parent_type, parent)
  }
end

def validate_field(context, ast_field, parent_type, parent)
  if parent_type.kind.union?
    context.errors << message("Selections can't be made directly on unions (see selections on #{parent_type.name})", parent, context: context)
    return GraphQL::Language::Visitor::SKIP
  end
  field =  parent_type.get_field(ast_field.name)
  if field.nil?
    context.errors << message("Field '#{ast_field.name}' doesn't exist on type '#{parent_type.name}'", parent, context: context)
    return GraphQL::Language::Visitor::SKIP
  end
end