Problem with has_and_belongs_to_many

rubyonrails-talk
1

hello,

I have an m:n relationsship (has_and_belongs_to_many) between the models CoreUser and CoreGroup. So from some rails experience I thought I'd be able to add a (user,group)-relation simply by doing agroup.core_users << auser.

Unfortunately the following test-case fails on the last assertion (unless I include the line that is commented out):

  def test_add_remove     someuser = CoreUser.create!(:username => 'test2s', ...)     ag = CoreGroup.find_by_path("/Administrators")     assert !ag.core_users.include?(someuser)     assert !someuser.core_groups.include?(ag)

    ag.core_users << someuser # someuser.core_groups << ag     assert ag.core_users.include?(someuser)     assert someuser.core_groups.include?(ag)   end

Shouldn't it work with the line being commented out (one direction)?

The models look like this:

class CoreUser < ActiveRecord::Base

  has_one :core_page

  has_many :core_artifacts   has_many :core_rating1s   has_many :core_rating2s   has_many :core_messages   has_many :core_widgets   has_and_belongs_to_many :core_groups

  validates_presence_of :username, :first_name, :last_name, :email_address, :password_hash   validates_uniqueness_of :username, :email_address   [...]

class CoreGroup < ActiveRecord::Base

  has_one :core_page

  has_and_belongs_to_many :core_users   acts_as_tree :order => :name # sort children by :name

  validates_presence_of :name   validates_uniqueness_of :name, :scope => :parent_id

thanks,

2

Felix Natter <felix.natter-HLWg83l9DMfnMc1oM6GxaBvVK+yQ3ZXh@public.gmane.org> writes:

hello,

I have an m:n relationsship (has_and_belongs_to_many) between the models CoreUser and CoreGroup. So from some rails experience I thought I'd be able to add a (user,group)-relation simply by doing agroup.core_users << auser.

Unfortunately the following test-case fails on the last assertion (unless I include the line that is commented out):

  def test_add_remove     someuser = CoreUser.create!(:username => 'test2s', ...)     ag = CoreGroup.find_by_path("/Administrators")     assert !ag.core_users.include?(someuser)     assert !someuser.core_groups.include?(ag)

    ag.core_users << someuser # someuser.core_groups << ag     assert ag.core_users.include?(someuser)     assert someuser.core_groups.include?(ag)   end

Shouldn't it work with the line being commented out (one direction)?

The models look like this:

class CoreUser < ActiveRecord::Base

  has_one :core_page

  has_many :core_artifacts   has_many :core_rating1s   has_many :core_rating2s   has_many :core_messages   has_many :core_widgets   has_and_belongs_to_many :core_groups

  validates_presence_of :username, :first_name, :last_name, :email_address, :password_hash   validates_uniqueness_of :username, :email_address   [...]

class CoreGroup < ActiveRecord::Base

  has_one :core_page

  has_and_belongs_to_many :core_users   acts_as_tree :order => :name # sort children by :name

  validates_presence_of :name   validates_uniqueness_of :name, :scope => :parent_id

here is some additional information from a console session:

3

AR is like that. if you set an association, rails does not magically update the other in memory object with the reverse association (but it is there in the database of course).

Fred

4

AR is like that. if you set an association, rails does not magically update the other in memory object with the reverse association (but it is there in the database of course).

I don't know if anyone mentioned to throw a 'reload()' call in somewhere...

5

"Phlip" <phlip2005@gmail.com> writes:

AR is like that. if you set an association, rails does not magically update the other in memory object with the reverse association (but it is there in the database of course).

I don't know if anyone mentioned to throw a 'reload()' call in somewhere...

That was it, thanks to both of you! So I should change this code:

clas CoreGroup...   def add(user)     core_users << user if !core_users.include?(user)   end

  def remove(user)     core_users.delete(user)   end

to this:

class CoreGroup...   def add(user)     core_users << user if !core_users.include?(user)     user.save     user.reload # make ActiveRecord update relation info   end

  def remove(user)     core_users.delete(user)     user.save     user.reload # make ActiveRecord update relation info   end

? Or is there a method to just reload the relation info?

thanks in advance,

6

? Or is there a method to just reload the relation info?

I have seen this around:

  user.core_groups(:reload)

If it does what it says, then it only repopulates the controlled array inside core_groups, without reloading the entire user object, or forcing a reload of everything else

Insert standard complaint here about how awesome ActiveRecord associations look just before you put a cycle in your code and discover the risk of dirty data bugs! -->

7

Strictly speaking, I don't believe that it's necessary to call save on the user object. I realize that you're probably doing this because the user object may contain unsaved data, but it brings a second purpose to your method.

Also, if you ever need to use the results of CoreGroup#add or CoreGroup#remove in subsequent processing then the results are going to be a little muddied. As coded you will always save/reload the user regardless of whether the association was added/deleted. Consequently, the method will always return the result of the user.reload call. You could get into confusing scenarios if the results conflicted (e.g., association creaated but reload failed, association failed but reload succeeded).

Maybe something like this:

def add(user)   return false if core_users.include?(user)   if core_users << user     user.core_groups.reload     return true   end   false end

For your primary purpose you just need to know that you can call 'reload' on the association proxy to reload only that collection. The rest of the code only guarantees that you get a pass/fail response related to adding/removing on the collection.