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Asked: 2026-06-06T12:19:37+00:00

I’m using DataMapper in a Rails project, and have found that calling to_json on

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I’m using DataMapper in a Rails project, and have found that calling to_json on either a model instance or collection results in weird behavior: either a circular reference error from JSON, or an infinite series of the same query.

Assuming this was a problem with my model definitions, I opened a Rails console and created the simplest possible model:

class Foo
  include DataMapper::Resource
  property :id, Serial
  property :name, String
end

Foo.auto_migrate!  # create db table `foos` for this model

After saving an instance of this model, I did this:

f = Foo.first
f.to_json

At this point, the process appears to hang. If I tail -f log/development.log, I see this query executing over and over:

SQL (0.084ms)  SELECT `id`, `name` FROM `foos` ORDER BY `id`

Ruby’s memory usage continues to grow until I interrupt the command or kill the Ruby process. This is the same problem I had with my actual models, so I don’t think it was related to a wrong model definition.

What causes this bizarre behavior?

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1 Answer

  1. Editorial Team

    It seems that the Object#to_json method, as implemented by ActiveSupport, serializes all the attributes of the object. Datamapper maintains a self-referential variable (@_repository) that sends the method into a recursive tailspin.

    Good news is that the dm-serializer gem (as you mentioned) provides the necessary functionality, and the Rails has already stated how to fix the problem in the dm-rails project (it just hasn’t happened yet).

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