Manipulate an array attribute using multiple check boxes

E.g. when you're using a tagging Archive gem, you have seen virtual attributes that get and set a string array:

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post = Post.last
puts post.tag_list # ['foo', 'bar', 'baz']
post.tag_list = ['bam']
puts post.tag_list # ['bam']

If you would like to create a form displaying one check box per tag, you can do this:

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- form_for @post do |form|
  = form.check_box :tag_list, { :multiple => true }, 'foo', nil
  = form.check_box :tag_list, { :multiple => true }, 'bar', nil
  = form.check_box :tag_list, { :multiple => true }, 'bam', nil
  = form.check_box :tag_list, { :multiple => true }, 'baz', nil

For a post with the tags "foo" and "bar", this will render the form like this:

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[x] foo
[x] bar
[ ] baz
[ ] bam

The check_box helper will check whether the given value is included in the tag_list array. It also survives form round trips as expected when validation errors occur.

If the form above is submitted, it will send the following params:

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{ :post => { :tag_list => ['foo', 'bar', '', '', ''] } }

Note how one empty string is sent for every check box. This is a feature which allows you to uncheck all checkboxes. Your tagging gem will usually filter those out automatically.

(Discovered by snatchev Archive on APIdock Archive )