Create relationships when scaffolding

Question

Hi I am new to Ruby on Rails. I am trying to create a small blog site. I have two tables Posts and Comments. Each Post will have many comments. I generate the tables using these commands.

rails g scaffold Post title:string body:text author:string
rails g scaffold Comment body:string author:string

Now I want to add the relationship to the model classes. I add has_many :comments to the Post class and belongs_to :post to the Comment class. However when I try to call post.comments I get a runtime error saying SQLException: no such column: comments.post_id. Should I create a migration and add post_id under Comment or is there a way to achieve this when scaffolding?

Could you change the accepted answer for this question as the currently accepted answer is out of date? Thx — JamieD, Oct 22 '15 at 15:16

score 156 · Accepted Answer · answered Oct 03 '14 at 10:07

156

Scaffold actually provides a way to generate relationships, you should use the :references data type

rails g scaffold Comment body:string author:string post:references

This will generate a migration for the comments table with a post_id field and index for it. The generator will also add belongs_to :post to the Comment model.

It will not however generate the reverse side of the relationship so you'll need to add

has_many :comments

to the Post model yourself. You will also need to add nested resource routing if this is something you need as the generator can not handle this.

answered Oct 03 '14 at 10:07

JamieD

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When you manually add the has_many field how do you generate the migration? – lapinkoira Apr 11 '16 at 12:08
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@lapinkoira In the above example, the `post:references` bit that's included when scaffolding the Comments model will add a foreign key on a Comment referencing the Post that it belongs to. The `has_many :comments` relationship needs to be added to the Post model but doesn't require an additional migration. – darkmoves May 30 '16 at 15:57

score 9 · Answer 2 · answered Nov 18 '12 at 22:44

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You are definitely on the right track. If you add the post_id column when generating the Comment scaffold your relationship will then work (although you still need to add the has_many :comments and belongs_to :post)

So the updated generator call would look like this:

rails g scaffold Comment body:string author:string post_id:integer

answered Nov 18 '12 at 22:44

agmin

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wouldn't it be better to use :foreign_key in this case as opposed to an :integer? – Arthur Collé Jan 22 '19 at 06:42

score 3 · Answer 3 · answered Dec 19 '18 at 06:32

3

You can also use belongs_to like this:

rails g scaffold Comment body:string author:string post:belongs_to

answered Dec 19 '18 at 06:32

Jeremy Lynch

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Create relationships when scaffolding

3 Answers3