ETag idea for GraphQL + Ruby

etag_middleware.rb

# An untested idea to use objects' `update_at` properties to determine
# whether the query response has changed since a given time. 
# 
# Requires some setup, you must pass the `updated_at` from the incoming ETag into the query: 
#     
# ```
# # The `context` object itself is read-only, so use a nested hash, which may be modified during query execution:
# etags = {updated_at: last_etag}
#
# result = MySchema.execute(query_string, context: {etags: etags})
#
# # The middleware will modify the hash you passed in:
# new_etag = etags[:updated_at]
# 
# if new_etag > last_etag 
#   # ... Profit!
# ```
# 
#
# When you define your schema, you could add this middleware: 
#
# ```
# MySchema.middleware << EtagMiddleware.new 
# ```
# 
# More about middlewares for the GraphQL gem: https://github.com/rmosolgo/graphql-ruby/blob/master/guides/defining_your_schema.md#middleware
#
class EtagMiddleware 
  def call(parent_type, parent_object, field_definition, field_args, query_context, next_middleware)
    # Resolve the field as normal: 
    resolved_object = next_middleware.call 
    
    # If the object can yield some kind of `updated_at`, maybe use it for the etag:
    new_updated_at = get_updated_at(resolved_object)
    if new_updated_at && new_updated_at > query_context[:etags][:updated_at]
      query_context[:etags][:updated_at] = new_updated_at
    end
    
    # Return the resolved object:
    resolved_object
  end
  
  private 
  
  def get_updated_at(resolved_object) 
    if resolved_object.respond_to?(:updated_at)
      resolved_object.updated_at 
    elsif resolved_object.is_a?(Array) # Extend this to handle other collection objects like `ActiveRecord::Relation`s
      resolved_object
        .map { |obj| get_updated_at(obj) }
        .compact 
        .max 
    else
      nil 
    end
  end
end