using include_blank in f.select but not working - ruby-on-rails

Question

I want to use dropdown to list all my projects and the first option is blank.
but when I add :include_blank => true, it doesn't work.
just like that:
<%= f.select(:project_id, :include_blank => true) do %>
<% #projects.each do |p| %>
<%= content_tag(:option, p.name, value: p.id) %>
<% end -%>
<% end -%>
but if I use
<%= f.select(:title, %w{ Male Female }, :include_blank => true) %>
It works exactly!
I don't know how to revise the first code to make the blank appear.
Thanks!

Answer 1

Use like this:
<%= f.select :category_id, nil, :include_blank => true %>
In the documention API LINK
select(object, method, choices = nil, options = {}, html_options = {}, &block)

Answer 2

I think the problem is the <%= content_tag … %>. Try this:
<%= f.select(:project_id, :include_blank => true) do
#projects.each do |p|
content_tag(:option, p.name, value: p.id)
end
end %>
(ie. just a single ERB output embedding tag)
Update: looking over this again, you are not really customising the way the options are generated, do you really need the block?
<%= f.select(:project_id, #projects.collect { |p| [p.name, p.id] }, :include_blank => true) %>

Answer 3

U can do this:
<%= f.select(:project, :id, #projects.collect {|p| [ p.name, p.id ] }, {:include_blank => true}) %>
Or
<%= f.select(:project_id, #projects.collect {|p| [ p.name, p.id ] }, {:include_blank => true}) %>
Or you can use prompt:
<%= f.select(:project, :id, #projects.collect {|p| [ p.name, p.id ] }, {:prompt => 'select project'}) %>