Getting checkboxes set up in Rails when using Materialize CSS can be tricky. You need to place the label tag after the checkbox tag and they must have the exact same name. Here is a slim code snippet that works:

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.row.margin
  .col.s12
    h6 Active in:
    - @tournament.rounds.each do |r|
      .col.s3
        = check_box_tag "round_status[#{ r.number }]", "active", true
        = label_tag "round_status[#{ r.number }]", "Round #{ r.number }"

This will generate the following HTML code:

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<div class="col s12">
  <h6>Active in:</h6>
  <div class="col s3">
    <input type="checkbox" name="round_status[1]" id="round_status_1" value="active" checked="checked">
    <label for="round_status_1">Round 1</label>
  </div>
  <div class="col s3">
    <input type="checkbox" name="round_status[2]" id="round_status_2" value="active" checked="checked">
    <label for="round_status_2">Round 2</label>
  </div>
  <div class="col s3">
    <input type="checkbox" name="round_status[3]" id="round_status_3" value="active" checked="checked">
    <label for="round_status_3">Round 3</label>
  </div>
  <div class="col s3">
    <input type="checkbox" name="round_status[4]" id="round_status_4" value="active" checked="checked">
    <label for="round_status_4">Round 4</label>
  </div>
</div>

The checkboxes are checked by default in this example due to the third argument set to true in the check_box_tag.

Notice how in each input tag id="round_status_*" matches the label tag for="round_status_*". These must be the same in order for this to work properly. If all four checkboxes are left in the default checked state the params will include:

"round_status"=>{"1"=>"active", "2"=>"active", "3"=>"active", "4"=>"active"}

If the user un-checks the first checkbox and submits the form the params will include:

 "round_status"=>{"2"=>"active", "3"=>"active", "4"=>"active"}

Take this into account when setting up the controller action…