在collection_select上为nil：NilClass定义未定义的方法`map'-调用的引用可能为nil

Question

After reading through the other SO answers, it's pretty clear that there are some common themes.

Mostly this type of error seems to happen when the object called is not defined yet, but in this case we have a has_many relationship that may not have a referenced entry when building the selection.

Class Tag
  property :name, type: String
  has_many :in, :tagged, type: :CONCEPTUAL_TAG, model_class: :Artefact
end

One option is to do something like this:

<div class="field">
  <%= f.label :tagged_id %><br>

  <% if !@tag.tagged.empty? %>
    <%= f.collection_select(:tagged, Artefact.all.sort { |a,b| a.name <=> b.name }, :id, :name, options = {:prompt => "Please Select an Item", :selected => @tag.tagged.map(&:id)}, html_options = {:multiple => true, :class=>"search"}) %>
  <% else %>
    <%= f.collection_select(:tagged, Artefact.all.sort { |a,b| a.name <=> b.name }, :id, :name, options = {:prompt => "Please Select an Item"}, html_options = {:multiple => true, :class=>"search"}) %>
  <% end %>
</div>

But this is definitely not DRY.

Is there a way to select nothing when there is no association, and pre-populate when there is while keeping to a single f.collection_select ?

Answer 1

You don't have to use the condition in your view. Even if there are no items in @tag.tagged , it is still an ActiveRecord::Relation instance, so you can go ahead and just call map on it:

<%= f.collection_select(:tagged, 
  Artefact.all.sort { |a,b| a.name <=> b.name }, 
  :id,
  :name,
  options = {
    :prompt => "Please Select an Item",
    :selected => @tag.tagged.map(&:id)
  },
  html_options = {
    :multiple => true,
    :class=>"search"
  }) 
%>