Error removing column Rails

In my Rails app, I have a model called Rutinas. After some time, I needed to add some columns to the table so I generated a migration 20171116094810_add_votos_y_veces_asignada_to_rutinas.rb:

class AddVotosYVecesAsignadaToRutinas < ActiveRecord::Migration[5.1]
  def change
    add_column :rutinas, :votos_pos, :integer, :default => 0
    add_column :rutinas, :votos_neg, :integer, :default => 0
    add_column :rutinas, :veces_asig, :integer, :default => 0
  end
end

After some other migrations, I needed to delete two columns that I don't need anymore votos_pos and votos_neg so I generated another migration 20171117092026_remove_votos_from_rutinas.rb:

class RemoveVotosFromRutinas < ActiveRecord::Migration[5.1]
  def change
    remove_column :rutinas, :votos_pos, :integer
    remove_column :rutinas, :votos_neg, :integer
  end
end

The problem is that when I run rails db:migrate to migrate this last migration, it throws some weird error:

== 20171117092026 RemoveVotosFromRutinas: migrating ===========================
-- remove_column(:rutinas, :votos_pos)
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:

SQLite3::ConstraintException: FOREIGN KEY constraint failed: DROP TABLE "rutinas"
C:/Users/pepe/Dropbox/pepe/KeepMeFit/KeepMeFit-git/db/migrate/20171117092026_remove_votos_from_rutinas.rb:3:in `change'
bin/rails:4:in `require'
bin/rails:4:in `<main>'

Caused by:
ActiveRecord::InvalidForeignKey: SQLite3::ConstraintException: FOREIGN KEY constraint failed: DROP TABLE "rutinas"
C:/Users/pepe/Dropbox/pepe/KeepMeFit/KeepMeFit-git/db/migrate/20171117092026_remove_votos_from_rutinas.rb:3:in `change'
bin/rails:4:in `require'
bin/rails:4:in `<main>'

Caused by:
SQLite3::ConstraintException: FOREIGN KEY constraint failed
C:/Users/pepe/Dropbox/pepe/KeepMeFit/KeepMeFit-git/db/migrate/20171117092026_remove_votos_from_rutinas.rb:3:in `change'
bin/rails:4:in `require'
bin/rails:4:in `<main>'
Tasks: TOP => db:migrate
(See full trace by running task with --trace)

My Rutinas model is the following:

class Rutina < ActiveRecord::Base    
  validates_presence_of :nombre

  belongs_to :user
  has_many :repeticions
  has_many :entrenos, dependent: :destroy
  has_many :votos

  has_many :days, dependent: :destroy

  def get_array_dias
    (1..self.repeticions.last.dia).to_a
  end

  def get_number_of_days
    days.count
  end

end

User, repeticion, entreno, voto and day are other tables which are non-related to the columns that I'm trying to delete. That is to say, these columns are not a foreign key and any foreign key references these columns.

回答1:

It seems like the problem is the limitation of SQLite. If you check the documentation you'll see, that it only allows adding a column, but not removing one: https://sqlite.org/lang_altertable.html

It is possible, that the migration actually drops the whole table and recreates it, when a column is removed. This would explain the DROP TABLE "rutinas" message. Of course if the while table is dropped, it would make sense that certain foreign key constraints fail.

回答2:

It will be fixed in Rails 6 including https://github.com/rails/rails/pull/32865 . This requires SQLite database 3.8 and higher. I do not think it will be back ported into Rails 5.2 or older.

My answer may not give you any practical feedback how to write your code, what I can say now is upgrading Rails 6 (release date is not fixed) may resolve this problem.