controllers.rb

def user_params
  params.fetch(:user, {}).permit(:id, :production_media_users_attributes: [:id, :user_id, {production_medium_id: []}])
end

def project_params
    params.fetch(:project, {}).permit(:id, genres_projects_attributes: [:id, :project_id, {genre_id: []}])
end

models.rb

class User < ActiveRecord::Base
	has_many :production_media, through: :production_media_users
	has_many :production_media_users
	accepts_nested_attributes_for :production_media_users
end

class ProductionMedium < ActiveRecord::Base
	has_many :users, through: :production_media_users
	has_many :production_media_users
end

class Project < ActiveRecord::Base
	has_many :genres, through: :genres_projects
	has_many :genres_projects
	accepts_nested_attributes_for :genres_projects
end


class Genre < ActiveRecord::Base
	has_many :projects, through: :genres_projects
	has_many :genres_projects
end
	

output

{"utf8"=>"✓", "_method"=>"patch", "project"=>{"id"=>"1913", "name"=>"Turquoise Girder", "genres_projects_attributes"=>{"genre_id"=>["11"]}, "logline"=>"", "synopsis"=>"", "imdb_link"=>"", "status"=>""}, "commit"=>"Update", "attribute"=>"profile", "controller"=>"projects", "action"=>"attribute_edit"}

{"utf8"=>"✓", "_method"=>"patch", "user"=>{"username"=>"STUDIOCOMPS", "given_name"=>"ALLEN", "family_name"=>"LEE", "production_media_users_attributes"=>[{"production_medium_id"=>["2"]}], "experience1"=>"Lions Gate Entertainment", "experience2"=>"", "education1"=>"Columbia MBA", "education2"=>"UC Berkeley", "linkedin_url"=>"http://www.linkedin.com/in/...", "bio"=>"..."}, "commit"=>"Update Profile", "action"=>"update", "controller"=>"users", "id"=>"91"}

views.erb.html

<%= select_tag 'user[production_media_users_attributes][][production_medium_id][]', options_for_select(ProductionMedium.order(:id).collect{|g| [g.name, g.id]}), { include_hidden: true, multiple: true,  required: true, placeholder: 'Choose One or More', class: 'form-control menu-select2'} %>
				
<%= 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,  required: true, placeholder: 'Choose One or More', class: 'form-control menu-select2'} %>