ruby on rails - Cocoon, nested fields inside nested fields -

i have project, project model, main groups model , trades model, each project can have many main groups, each main group can have many trades.

i can create project , main groups trades not saving.

project.rb:

class project < activerecord::base     belongs_to :user      has_many :consultants     accepts_nested_attributes_for :consultants, :reject_if => :all_blank, :allow_destroy => true      has_many :contractors     accepts_nested_attributes_for :contractors, :reject_if => :all_blank, :allow_destroy => true      has_one :client     accepts_nested_attributes_for :client      has_many :main_groups     accepts_nested_attributes_for :main_groups, :reject_if => :all_blank, :allow_destroy => true      has_many :trades, through: :main_groupsp end 

trade.rb

class trade < activerecord::base   belongs_to :main_group end 

main_group.rb

class maingroup < activerecord::base   belongs_to :project    has_many :trades   accepts_nested_attributes_for :trades, :reject_if => :all_blank, :allow_destroy => true end 

main group partial:

.nested-fields.add-tasks     .row         .col-md-12             .form-horizontal                 .panel.panel-default                     .panel-heading                          main group                     .panel-body                         .form-group                             = f.label :name, :class => 'col-sm-1 control-label'                             .col-sm-11                                 = f.input :name,:label => false, input_html:{class: "form-input form-control"}                         .form-group                             = f.simple_fields_for :trades |trade|                                 = render 'projects/forms/trade_fields', f: trade                         .trade-links.form-group                             .col-md-12                                 = link_to_add_association 'add trade', f, :trades,  :partial => 'projects/forms/trade_fields', class: "btn btn-default add-button pull-right btn-success"                         .form-group                             .col-sm-12                                 = link_to_remove_association 'remove main group', f,class: "btn btn-default pull-right btn-danger" 

trade form partial:

.nested-fields.add-tasks     .row         .col-md-12             .form-horizontal                 .panel.panel-default                     .panel-heading                          trade                     .panel-body                         .form-group                             = f.label :name, :class => 'col-sm-1 control-label'                             .col-sm-11                                 = f.input :name,:label => false, input_html:{class: "form-input form-control"}                         .form-group                             = f.label :cost, :class => 'col-sm-1 control-label'                             .col-sm-11                                 = f.input :cost,:label => false, input_html:{class: "form-input form-control"}                         .form-group                             = f.label :start_date, :class => 'col-sm-1 control-label'                             .col-sm-11                                 = f.input :start_date,:label => false, input_html:{class: "form-input form-control"}                         .form-group                             = f.label :end_date, :class => 'col-sm-1 control-label'                             .col-sm-11                                 = f.input :end_date,:label => false, input_html:{class: "form-input form-control"}                         .form-group                             .col-sm-12                                 = link_to_remove_association 'remove trade', f,class: "btn btn-default pull-right btn-danger"