_form.html.erb

<%= form_for Group.new do |f| %>
    <table summary = "groups from trainer">
      <tr>
        <th><%= f.label(:name) %></th>
        <td><%= f.text_field(:name) %></td>
      </tr>
      <tr>
        <th><%= f.label(:area) %></th>
        <td><%= f.text_field(:area) %></td>
      </tr>
      <div class="actions">
        <%= f.submit %>
      </div>
<% end %>
</table>

trainers_controller.rb

class GroupsController < ApplicationController
  #before_action :set_group, only: [:show, :edit, :update, :destroy]
 before_action :set_trainer
  # GET /groups
  # GET /groups.json
  def index
    @groups = Group.all
  end

  # GET /groups/1
  # GET /groups/1.json
  def show
  end

  # GET /groups/new
  def new
    @trainer = Trainer.find(params[:trainer_id])
  end

  # GET /groups/1/edit
  def edit
  end

  # POST /groups
  # POST /groups.json
  def create
    @group = Group.new(group_params)

    respond_to do |format|
      if @group.save
        format.html { redirect_to @group, notice: 'Group was successfully created.' }
        format.json { render action: 'show', status: :created, location: @group }
      else
        format.html { render action: 'new' }
        format.json { render json: @group.errors, status: :unprocessable_entity }
      end
    end
  end

  # PATCH/PUT /groups/1
  # PATCH/PUT /groups/1.json
  def update
    respond_to do |format|
      if @group.update(group_params)
        format.html { redirect_to @group, notice: 'Group was successfully updated.' }
        format.json { head :no_content }
      else
        format.html { render action: 'edit' }
        format.json { render json: @group.errors, status: :unprocessable_entity }
      end
    end
  end

  # DELETE /groups/1
  # DELETE /groups/1.json
  def destroy
    @group.destroy
    respond_to do |format|
      format.html { redirect_to groups_url }
      format.json { head :no_content }
    end
  end

  private
    # Use callbacks to share common setup or constraints between actions.
    def set_group
      @group = Group.find(params[:id])
    end

  def set_trainer
  @trainer = Trainer.find(params[:trainer_id])
  end
  def find_trainer
    if params[:trainer_id]
      @trainer = Trainer.find(params[:trainer_id]).permit(:trainer_id)
    end

    # Never trust parameters from the scary internet, only allow the white list through.
    def group_params
      params.require(:group).permit(:name, :area, :trainer_id)
    end
  end
end