undefined methods form [NEWBE] - ruby-on-rails

Question

So I have a form and corresponding classes. I got an error:
undefined method `start' for #<Klass id: nil, name: nil, teacher: nil, day: nil>
for line: <%= f.text_field :start, class: 'form-control' %>
and (when I'm trying to delete the above one) <%= f.text_field :duration, class: 'form-control' %>
removing both fields makes my website ok.
My whole form code:
<%= form_for #klass do | f | %>
<div class = “form-group”>
<div class="form-group">
<%= f.label :name %>
<%= f.text_field :name, class: 'form-control' %>
</div>
<div class="form-group">
<%= f.label :teacher %>
<%= f.text_field :teacher, class: 'form-control' %>
</div>
<div class="form-group">
<%= f.label :start %>
<%= f.text_field :start, class: 'form-control' %>
</div>
<div class="form-group">
<%= f.label :duration %>
<%= f.text_field :duration, class: 'form-control' %>
</div>
<div class="form-group">
<%= f.label :day %>
<%= f.select :day, ['MON', 'TUE', 'WED', 'THU', 'FRI', 'SAT', 'SUN'] %>
</div>
<%= f.submit 'Submit', class: 'btn btn-default' %>
</div>
<% end %>
model (changing integer to string doesn't make any difference, but that is the only difference betweenthese two fields and the rest of the form that I can see) :
class Klass < ActiveRecord::Base
validates :name, presence: true
validates :teacher, presence: true
validates :day, presence: true
validates :start, presence: true
validates :duration, presence: true, numericality: { only_integer: true }
end
database file:
class CreateKlasses < ActiveRecord::Migration[5.0]
def change
create_table :klasses do |t|
t.string :name
t.string :teacher
t.string :day
t.integer :start
t.integer :duration
end
end
end
and controller:
class KlassesController < ApplicationController
def new
#klass = Klass.new
end
end
Looks for me like I missed to declare these two form fields but where else I can look for it?

Answer 1

1- rake db:rollback
make sure you have these fields added in migration file
class CreateKlasses < ActiveRecord::Migration[5.0]
def change
create_table :klasses do |t|
t.string :name
t.string :teacher
t.string :day
t.integer :start
t.integer :duration
end
end
end
2- rake db:migrate
now reload rails console
reload!
or just close rails console and open rails console again.
check again Klass.new if it has all field that you added in migration. if those field exists then restart server and thats it.