<%= form.label :statistical_data_set_document_ids, 'Related statistical data sets' %>
<%= form.select :statistical_data_set_document_ids, options_from_collection_for_select(statistical_data_set_options, :first, :last, edition.statistical_data_set_document_ids), {}, multiple: true, class: 'chzn-select', data: { placeholder: "Choose statistical data sets..."} %>

<%= form.label :statistical_data_set_document_ids, 'Related statistical data sets' %>
<%= form.select :statistical_data_set_document_ids,
                options_from_collection_for_select(statistical_data_set_options, :first, :last, edition.statistical_data_set_document_ids),
                { include_blank: true },
                multiple: true,
                class: 'chzn-select',
                data: { placeholder: "Choose statistical data sets..."} %>

Add blank option to statistical data sets
Add blank option to statistical data sets
