accept_nested_attributes_for in a many-to-many relationship - ruby-on-rails

Question

I have tried to find a solution for this but most of the literature around involves how to create the form rather than how to save the stuff in the DB. The problem I am having is that the accepts_nested_attributes_for seems to work ok when saving modifications to existing DB entities, but fails when trying to create a new object tree.
Some background. My classes are as follows:
class UserGroup < ActiveRecord::Base
has_many :permissions
has_many :users
accepts_nested_attributes_for :users
accepts_nested_attributes_for :permissions
end
class User < ActiveRecord::Base
has_many :user_permissions
has_many :permissions, :through => :user_permissions
belongs_to :user_group
accepts_nested_attributes_for :user_permissions
end
class Permission < ActiveRecord::Base
has_many :user_permissions
has_many :users, :through => :user_permissions
belongs_to :user_group
end
class UserPermission < ActiveRecord::Base
belongs_to :user
belongs_to :permission
validates_associated :user
validates_associated :permission
validates_numericality_of :threshold_value
validates_presence_of :threshold_value
default_scope order("permission_id ASC")
end
The permission seem strange but each of them has a threshold_value which is different for each user, that's why it is needed like this.
Anyway, as I said, when I PUT an update, for example to the threshold values, everything works ok. This is the controller code (UserGroupController, I am posting whole user groups rather than one user at a time):
def update
#ug = UserGroup.find(params[:id])
#ug.update_attributes!(params[:user_group])
respond_with #ug
end
A typical data coming in would be:
{"user_group":
{"id":3,
"permissions":[
{"id":14,"name":"Perm1"},
{"id":15,"name":"Perm2"}],
"users":[
{"id":7,"name":"Tallmaris",
"user_permissions":[
{"id":1,"permission_id":14,"threshold_value":"0.1"},
{"id":2,"permission_id":15,"threshold_value":0.3}]
},
{"name":"New User",
"user_permissions":[
{"permission_id":14,"threshold_value":0.4},
{"permission_id":15,"threshold_value":0.2}]
}]
}
}
As you can see, the "New User" has no ID and his permission records have no ID either, because I want everything to be created. The "Tallmaris" user works ok and the changed values are updated no problem (I can see the UPDATE sql getting run by the server); on the contrary, the new user gives me this nasty log:
[...]
User Exists (0.4ms) SELECT 1 AS one FROM "users" WHERE "users"."name" = 'New User' LIMIT 1
ModelSector Load (8.7ms) SELECT "user_permissions".* FROM "user_permissions" WHERE (user_id = ) ORDER BY permission_id ASC
PG::Error: ERROR: syntax error at or near ")"
The error is obviously the (user_id = ) with nothing, since of course the user does not exists, there are no user_permissions set already and I wanted them to be created on the spot.

Answer 1

Thanks to looking around to this other question I realised it was a problem with the validation on the user.
Basically I was validating that the threshold_values summed up within certain constraints but to do that I was probably doing something wrong and Rails was loading data from the DB, which was ok for existing values but of course there was nothing for new values.
I fixed that and now it's working. I'll leave this here just as a reminder that often a problem in one spot has solutions coming from other places. :)