Validate using one GraphQL schema, but execute using another schema

validate_schema_example.rb

require "bundler/inline"

gemfile do
  gem "graphql", "2.2.6"
end

class NewSchema < GraphQL::Schema
  class Query < GraphQL::Schema::Object
    field :old_field, Integer
    def old_field = 1

    field :new_field, Integer
    def new_field = 2
  end

  query(Query)
end

OldSchema = GraphQL::Schema.from_definition <<-SCHEMA
type Query {
  oldField: Int
}
SCHEMA


def run_as_if_old_schema(query_str)
  # IRL this would be in your controller, for example
  validation_errs = OldSchema.validate(query_str)
  if validation_errs.any?
    {
      errors: validation_errs.map(&:to_h)
    }
  else
    NewSchema.execute(query_str, validate: false)
  end
end


pp run_as_if_old_schema("{ oldField }").to_h
# {"data"=>{"oldField"=>1}}
pp run_as_if_old_schema("{ newField }").to_h
# {:errors=>
#   [{"message"=>"Field 'newField' doesn't exist on type 'Query'",
#     "locations"=>[{"line"=>1, "column"=>3}],
#     "path"=>["query", "newField"],
#     "extensions"=>{"code"=>"undefinedField", "typeName"=>"Query", "fieldName"=>"newField"}}]}