Nested form don't save

I'm new do rails and MVC. I'm trying to save to my database a nested form that I made using simple-form and nested-form-for gems. When I submit the form only the 'parent' model is created and saved. Thankyou very much.

'parent model':

class Diet < ApplicationRecord
    belongs_to :coach

    has_many :user_eat_diets
    has_many :user, through: :user_eat_diets
    accepts_nested_attributes_for :user_eat_diets

    has_many :diet_composes
    has_many :foods, through: :diet_composes
    accepts_nested_attributes_for :diet_composes
end

'child' model:

class DietCompose < ApplicationRecord
    belongs_to :diet
    belongs_to :food
end

'parent' controller:

class DietsController < ApplicationController
    def new
        @diet = Diet.new
        @diet.diet_composes.build
    end

    def create
        @diet = Diet.new(diet_params)
        if @diet.save
            flash[:success] = "success"
        end
    end
    def diet_params
      params.require(:diet).permit(:name, :coach_id, :diet_composes_attributes)
    end

end

'child' controller:

class DietComposesController < ApplicationController
    def new
        @diet_compose = Diet_compose.new
    end

    def create
        @diet_compose = Diet_compose.new(diet_compose_params)
        if @diet_compose.save
            flash[:success] = "success"
        end
    end

    def diet_compose_params
      params.require(:diet_compose).permit(:quantity, :hour, :day, :food_id, :diet_id)
    end
end

the form view:

<%= simple_form_for @diet, :html => {:class => 'form-basic' }  do |f| %>
            <%= f.input :name %>
            <%= f.input :coach_id %>
            <%= f.nested_fields_for :diet_composes do |ff| %>
                    <%= ff.remove_nested_fields_link %>
                    <%= ff.input :hour %>
                    <%= ff.input :day %>
                    <%= ff.input :food_id %>
                    <%= ff.input :diet_id %>
                <% end %>
            <%= f.add_nested_fields_link :diet_composes %>
            <%= f.button :submit %>
        <% end %>

Also, when I do the command

Diet_compose.all

on rails console I got the error

LoadError: Unable to autoload constant Diet_compose, expected /home/tanaka/Desktop/E-xercite/app/models/diet_compose.rb to define it from (irb):8

You need to correct multiple things here. I have added the comments inline.

in Diet model (diet.rb)

class Diet < ApplicationRecord
  attr_accessible :diet_composes # make the attributes accessible for mass assignment.
end

In controllers:

class DietsController < ApplicationController
    def new
        @diet = Diet.new
        @diet.diet_composes.build
    end

    def create
        @diet = Diet.new(diet_params)
        if @diet.save
            flash[:success] = "success"
        end
    end
    def diet_params
       # add attributes of nested association to whitelist
      params.require(:diet).permit(:name, :coach_id, :diet_composes_attributes => [:hour, :day, :food_id, :diet_id])
    end

end

You don't really need DietComposesController's actions to get your nested attribute of Diet working. But still, correcting the errors below which are giving you LoadError

class DietComposesController < ApplicationController
    def new
        @diet_compose = DietCompose.new # Model name is always CamelCase. DietCompose rather than Diet_compose
    end

    def create
        @diet_compose = DietCompose.new(diet_compose_params) # here as well
        if @diet_compose.save
            flash[:success] = "success"
        end
    end

    def diet_compose_params
      params.require(:diet_compose).permit(:quantity, :hour, :day, :food_id, :diet_id, )
    end
end

Your command should be DietCompose.all. Even look at the definition of diet_model.rb. It declares the name as DietModel.

Your view seems fine. I haven't worked with nested_form_fields gem, so can't be 100% sure. Simple rule of debugging, see the rails server's log and infer :)