Question: Hi I have `has_and_belongs_to_many` relationship between Posts and Comments and in my edit action I have form that returns me list of ids `1,3,5,8` etc. I want build relationship between my current model and all models which ids are in the list so `@post.comments` will return Comments with `1,3,5,8` ids

In fact I need execute 

```
DELETE FROM comments_posts
WHERE post_id = @post.id

INSERT INTO comments_posts VALUES 
(1,@post.id)
(3,@post.id)
(5,@post.id)
(8,@post.id)

```

or do something similar

Answer: 
```
params[:list] ||= []                # Create an empty array if form is empty
@posts.comment_ids = params[:list]  # Recreate comment associations

```

See also the [Railscast about HABTM Checkboxes](http://railscasts.com/episodes/17-habtm-checkboxes).
Maybe you are looking something like this.

```
list = params[:list].join(',') # Assumes space separated to begin with
@posts.comments.find(:conditions => ["id IN (?)", list])

```

**UPDATE**

**Aside**: I am surprised by the *HABTM* relationship in your application. Do you really mean that a post can have many comments **and** a comment can belong to many posts? Surely a comment belongs to only one post, no?

Nevertheless, I believe you can achieve what you're looking for as follows (using your examples):

```
@post.comments.destroy_all

@post.comments.create(...)

```

It's just basic associations in ActiveRecord