Rails multiple select form: no implicit conversion of String into Integer - ruby-on-rails-4

Question

In Rails 4, Why isn't an array of genre_id being saved to the database? I think it's because the ids are being read as a string, when it needs to be saved as an integer, but in the past I didn't have this problem.
I get a no implicit conversion of String into Integer error when trying to POST the following:
<%= form_for #project do |f| %>
<%= select_tag 'project[genres_projects_attributes][genre_id][]',
options_for_select(Genre.order(:id).collect{|g| [g.name, g.id]}),
{ include_hidden: true, multiple: true, class: 'form-control'} %>
<% end %>
In my app,
Project has_many :genres, through: :genres_projects
My parameters look like this:
{"utf8"=>"✓","authenticity_token"=>"[xxx]",
"project"=>{"name"=>"sdfds",
"genres_projects_attributes"=>{"genre_id"=>["2",
"3",
"4"]},
"commit"=>"Go!"}
My project_params:
def project_params
params.require(:project).permit(:id, genres_projects_attributes:
[:id, {genre_id: []}, :project_id])
end
Here's the line in my controller that's throwing the error:
def create
#project = Project.create(project_params)
end

Answer 1

Unfortunately didn't find an answer so did the following workaround:
#genre_ids = project_params[:genres_projects_attributes][:genre_id]
#genre_ids.each do |g|
GenresProject.create!(project_id: #project.id, genre_id: g)
end
UPDATE:
I found out that it was the first blank value that was causing the issue. As of Rails 4.x, you can also add include_hidden: false in your form.
UPDATE AGAIN:
if project_params[:genres_projects_attributes]
#project.genres.delete_all
project_params[:genres_projects_attributes][:genre_id].each do |g|
#project.genres << Genre.find(g)
end
#project.update!(project_params.except :genres_projects_attributes)
else
#project.update!(project_params)