Assigning a GraphQL-Ruby changeset version from a directive in the query

changeset_directive.rb

require "bundler/inline"

gemfile do
  gem "graphql", "2.4.8", source: "https://rubygems.org"
  gem "graphql-enterprise", source: "https://gems.graphql.pro"
end

class MySchema < GraphQL::Schema
  class SwitchFields < GraphQL::Enterprise::Changeset
    release "2025-02-01"
  end

  class BaseField < GraphQL::Schema::Field
    include GraphQL::Enterprise::Changeset::FieldIntegration
  end

  class Query < GraphQL::Schema::Object
    field_class BaseField
    field :old_field, String, fallback_value: "Old", removed_in: SwitchFields
    field :new_field, String, fallback_value: "New", added_in: SwitchFields
  end

  # This directive definition is used for validation.
  class Changeset < GraphQL::Schema::Directive
    locations(QUERY, SUBSCRIPTION, MUTATION)
    argument :version, String
  end

  # This trace module will read the value from the query AST
  # and apply it to the `query.context` Hash.
  #
  # (Directive classes have a `.resolve` method, but it's too late for this
  # application because it's called during execution, but we need this
  # key to be present in context during _validation_.)
  #
  # `execute_multiplex` is the top-level hook, so we can do it there.
  module SetChangesetFromDirective
    def execute_multiplex(multiplex:)
      multiplex.queries.each do |query|
        operation = query.selected_operation
        if (dir = operation.directives.find { |d| d.name == "changeset" })
          changeset_version = dir.arguments.first.value
          query.context[:changeset_version] = changeset_version
        end
      end
      super
    end
  end

  query(Query)
  directive(Changeset)
  trace_with(SetChangesetFromDirective)
  use GraphQL::Schema::Visibility
  use GraphQL::Enterprise::Changeset::Release, changesets: [SwitchFields]
end

# Using context:
pp MySchema.execute("{ newField }", context: { changeset_version: "2025-02-01" }).to_h
# {"data" => {"newField" => "New"}}
pp MySchema.execute("{ oldField }", context: { changeset_version: "2025-02-01" }).to_h
# {"errors" => [{"message" => "Field 'oldField' doesn't exist on type 'Query'", "locations" => [{"line" => 1, "column" => 3}], "path" => ["query", "oldField"], "extensions" => {"code" => "undefinedField", "typeName" => "Query", "fieldName" => "oldField"}}]}
pp MySchema.execute("{ oldField }", context: { changeset_version: "2024-02-01" }).to_h
# {"data" => {"oldField" => "Old"}}

# Using the directive in a multiplex:
res = MySchema.multiplex(
  [
    { query: "query @changeset(version: \"2025-02-01\") { newField }" },
    { query: "query @changeset(version: \"2025-02-01\") { oldField }" },
    { query: "query @changeset(version: \"2024-02-01\") { oldField }" },
  ]
)

pp res.map(&:to_h)
# [
#   {"data" => {"newField" => "New"}},
#   {"errors" => [{"message" => "Field 'oldField' doesn't exist on type 'Query'", "locations" => [{"line" => 1, "column" => 43}], "path" => ["query", "oldField"], "extensions" => {"code" => "undefinedField", "typeName" => "Query", "fieldName" => "oldField"}}]},
#   {"data" => {"oldField" => "Old"}}
# ]

puts MySchema.to_definition(context: { changeset_version: "2025-02-01" })
# directive @changeset(version: String!) on MUTATION | QUERY | SUBSCRIPTION

# type Query {
#   newField: String
# }