Rails: Getting an array of object ids by query/conversion (for comparision)

Question

Basically, I want an array of ids from the database.

Some background:

• I'm rendering a list of objets (foos) to the user with a checkbox.
• If the user checks the box a row is create in a different table (bar)
• when rendering the foo list + checkbox I want to check if the unique id any given foo already exists in the bar id array.

I assumed that getting an array of ids would be much more efficient than querying the database for each instance of foo when rendering the foo list with checkboxes.

Ideas? Apologies in advance if this is not clear.

Answer 1

What's the problem, than? If you have rails has_many or has_and_belongs_to_many association, rails will fetch ids for you. See relevant tutorial for details, in short it's like @order_ids = @customer.order_ids.

Otherwise, you can easily use plain sql (or active record query).

Answer 2

I used the map method:

@bars = Bar.all(:select => bar.id)
@bars = @bars.map{|bar| bar.id}

Then I end up with array of ids. And only one query.