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

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开发者_JAVA技巧? Apologies in advance if this is not clear.

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we can use the map method also this way

Model.all.map(&:field_name)

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More recent versions of Rails support Model.pluck(:field_name)

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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.

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array1 = User.all.collect(&:id)

The array1 has the values of all the ids in the table Users.

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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).