# Habtm and collection\_select question **URL:** \ **Category:** Rails\ **Created:** [December 8, 2005, 4:10pm UTC](https://www.ruby-forum.com/t/habtm-and-collection-select-question/49764 "2005-12-08T16:10:07Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![zerohalo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/5fc32e/32.png) [@zerohalo](https://www.ruby-forum.com/u/zerohalo)\ **Post date:** [December 8, 2005, 4:10pm UTC](https://www.ruby-forum.com/t/habtm-and-collection-select-question/49764/1 "2005-12-08T16:10:07Z")
I apologize in advance for my ignorance, but I’m stuck with a problem and after a couple of hours of looking for answers in the documentation and help, I’m still stuck so I was wondering if someone might have the answer. I have tables Projects and Stages, each with an habtm relationship to each other, and a projects\_stages table which contains projects\_id, stages\_id and a date field. (A project can have any number of stages, and completed stages are recorded in project\_stages.) On …/projects/edit I display the completed steps for the project (all records in projects\_stages which have project\_id for the selected project). So far so good. Now I want a drop-down field to select the next step and write a new record to the projects\_stages field. So I put the following: \<%= collection\_select(:project, :stages, Stage.find\_all, :id, :description, {:include\_blank =\> true}) %\> The select box populates correctly, but when saving I get an error that Stage expected but received String. The error page shows that the stage\_id is being passed back to the controller, but in this instance what I need is to add a row to the project\_stages table containing both the project\_id and the stages\_id. Does Rails ‘magically’ do this (like it does with a has\_many relationship) and I’m just using collection\_select incorrectly, or is there some other way I’m supposed to do this? I’m not sure what is the proper approach in this case. Thanks. ---
**Author:** ![zerohalo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/5fc32e/32.png) [@zerohalo](https://www.ruby-forum.com/u/zerohalo)\ **Post date:** [December 8, 2005, 11:24pm UTC](https://www.ruby-forum.com/t/habtm-and-collection-select-question/49764/2 "2005-12-08T23:24:36Z")
zero halo wrote: > \<%= collection\_select(:project, :stages, Stage.find\_all, :id, > :description, {:include\_blank =\> true}) %\> > > The select box populates correctly, but when saving I get an error that > Stage expected but received String. The error page shows that the If I understand you correctly, the user is selecting the step that they’ve just completed, to be added to the project.stages completed list. In this case I’d recommend you create an attr\_accessor in your project model called newly\_completed\_stage\_id, then use \<%= collection\_select(:project, :newly\_completed\_stage\_id, Stage.find\_all - @project.stages, :id, :description, {:include\_blank =\> true}) %\> and have a before\_save method in Projects to add any new stage: stages.push\_with\_attributes( Stage.find(newly\_completed\_stage\_id.to\_i), :date\_completed =\> Date.today ) unless newly\_completed\_stage\_id.blank? – We develop, watch us RoR, in numbers too big to ignore. ---
**Author:** ![zerohalo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/5fc32e/32.png) [@zerohalo](https://www.ruby-forum.com/u/zerohalo)\ **Post date:** [December 9, 2005, 5:05am UTC](https://www.ruby-forum.com/t/habtm-and-collection-select-question/49764/3 "2005-12-09T05:05:57Z")
mrj wrote: > In this case I’d recommend you create an attr\_accessor in your > project model called newly\_completed\_stage\_id, then use > > \<%= collection\_select(:project, :newly\_completed\_stage\_id, > Stage.find\_all - @project.stages, :id, :description, {:include\_blank =\> > true}) %\> > > and have a before\_save method in Projects to add any new stage: > > stages.push\_with\_attributes( > Stage.find(newly\_completed\_stage\_id.to\_i), :date\_completed =\> Date.today > ) unless newly\_completed\_stage\_id.blank? Awesome, thanks! I must say, I have a hard time wrapping my head around some of the Rails behind-the-scenes magic, such as understanding exactly how push\_with\_attributes knows to add a new record to my project\_stages join table, but hey, it works! I should have gotten it from reading the Agile book, but my brain’s a bit foggy these days. This might be helpful to post on the Rails wiki. I’ll attempt a write-up though I’m not sure I can explain it all like it should. I also liked the ‘Stage.find\_all - @project.stages’ little bit of code. I didn’t realize you could subtract one collection from another like that. Very cool. ---
**Author:** ![zerohalo](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/z/5fc32e/32.png) [@zerohalo](https://www.ruby-forum.com/u/zerohalo)\ **Post date:** [December 9, 2005, 10:36am UTC](https://www.ruby-forum.com/t/habtm-and-collection-select-question/49764/4 "2005-12-09T10:36:37Z")
zero halo wrote: > to post on the Rails wiki. I’ll attempt a write-up though I’m not sure I > can explain it all like it should. The job of push\_with\_attributes is to add an object to the list of objects in a HABTM association. To do this it must add a record to the join table, geting the project\_id from the Project object on which stages is called, the stage\_id from the first parameter Stage object, and the attributes associated with the join from the second parameter. The stage object itself is not saved because it is not a new record. > I also liked the ‘Stage.find\_all - @project.stages’ little bit of code. > I didn’t realize you could subtract one collection from another like > that. Very cool. That works because Rails cleverly uses an AR object’s id attribute for equality comparison. One other thing: find\_all is depricated, use find(:all) instead. – We develop, watch us RoR, in numbers too big to ignore.