`stack level too deep` when overwriting `to_json`

Rails
1

I’m using
acts_as_commentable_with_threading,
and it worked fine until I tried to overwrite the to_json function to
include more details about the associated user.

Here’s part of the comment class (which was originally defined in the
plugin, but I moved it to the models folder, a move which seems not to
affect the error):

def to_json(options = {})
  {
    :id => id,
    :body => body,
    :parent_id => parent_id,
    :created_at => created_at,
    :updated_at => updated_at,
    :user => user ### the line in question
  }.to_json(options)
end

When the line is included, the server responds accordingly the first
time you request the resource, but every subsequent request (a
particular oddity), I get:

SystemStackError (stack level too deep):
app/models/comment.rb:21:in to_json' C:1:in send’
C:1:in encode' C:1:in send’
C:1:in encode' app/models/item.rb:61:in to_json’
app/controllers/items_controller.rb:20
app/controllers/items_controller.rb:18:in show' C:/Ruby/lib/ruby/1.8/webrick/httpserver.rb:104:in service’
C:/Ruby/lib/ruby/1.8/webrick/httpserver.rb:65:in run' C:/Ruby/lib/ruby/1.8/webrick/server.rb:173:in start_thread’
C:/Ruby/lib/ruby/1.8/webrick/server.rb:162:in start' C:/Ruby/lib/ruby/1.8/webrick/server.rb:162:in start_thread’
C:/Ruby/lib/ruby/1.8/webrick/server.rb:95:in start' C:/Ruby/lib/ruby/1.8/webrick/server.rb:92:in each’
C:/Ruby/lib/ruby/1.8/webrick/server.rb:92:in start' C:/Ruby/lib/ruby/1.8/webrick/server.rb:23:in start’
C:/Ruby/lib/ruby/1.8/webrick/server.rb:82:in `start’

Removing the line prevents the error from showing up.

I call associations in to_json elsewhere in the application with no
issues.

2

Steven Xu wrote:

def to_json(options = {})
  {
    :id => id,
    :body => body,
    :parent_id => parent_id,
    :created_at => created_at,
    :updated_at => updated_at,
    :user => user ### the line in question
  }.to_json(options)
end

Let’s think about what the call to to_json on the Hash is doing…

Each key/value pair will be sent the message to_json. I assume the Hash
takes care of providing the JSON representation of the more basic types
like String, Fixnum, etc. But, what about when to_json gets sent to the
user object.

Given that Item has_one :user then likely User has_many :items. The item
your working with may be contained within an array of user, if user’s
to_json includes items. This might explain how you end up with a “stack
level too deep.”

Since I don’t know how your User model is behaving I can’t say for sure
that is the problem, but it is something to consider.

3

Robert W. wrote:

Each key/value pair will be sent the message to_json. I assume the Hash
takes care of providing the JSON representation of the more basic types
like String, Fixnum, etc. But, what about when to_json gets sent to the
user object.

Given that Item has_one :user then likely User has_many :items. The item
your working with may be contained within an array of user, if user’s
to_json includes items. This might explain how you end up with a “stack
level too deep.”

Since I don’t know how your User model is behaving I can’t say for sure
that is the problem, but it is something to consider.

Thanks for the helpful description. I have made that mistake before (not
with cross-model associations, which are pretty easy to catch, but with
a nested set when I tried to establish aunt and uncle relationships),
and I am sure it’s not the case here. Typically, I force jsonning
associations to be manually declared in the options rather than enable
them by default, but I did it here because I ruled it out as a cause.
For the record, the to_json instance method on User doesn’t invoke any
associations.

I am at a loss of how to troubleshoot this because:

(1) The “works once then fails thereafter” behaviour is really weird.
(2) I can’t reproduce the error through the console (which produces the
expected JSON output, user and all)

4

On Jul 14, 10:48 pm, Steven Xu [email protected] wrote:

Robert W. wrote:

I am at a loss of how to troubleshoot this because:

(1) The “works once then fails thereafter” behaviour is really weird.

Does it only happen when class caching is turned off ? (don’t forget
to restart the server after you change that)

Fred