Rails removing a column from activerecord not working - sql

Question

I want to remove container:references from my table, I have tried:
rails generate migration RemoveContainerfromCreateTasks container:references
followed by rails db:migrate, but it my reference field is still not removed.
Below is my ActiveRecord
class CreateTasks < ActiveRecord::Migration[6.1]
def change
create_table :tasks do |t|
t.string :title
t.text :body
t.references :container, null: false, foreign_key: true
t.text :tag
t.datetime :due
t.integer :priority
t.timestamps
end
end
end
class RemoveContainerfromCreateTasks < ActiveRecod::Migration[6.1]
def change
end
end

Answer 1

The issue here is really a sneaky capitalization error. Running:
rails generate migration RemoveContainerfromCreateTasks container:references
Will generate a migration with an empty change block which will do absolutely nothing when you migrate it except modify the migrations meta table (a table that AR uses to keep track of which migrations have been run). But if you properly capitalize From:
rails generate migration RemoveContainerFromCreateTasks container:references
It will generate:
class RemoveContainerFromCreateTasks < ActiveRecord::Migration[6.0]
def change
remove_reference :create_tasks, :container, null: false, foreign_key: true
end
end
Rails isn't actually intelligent. It just casts the name argument into snake case and compares it to a set of patterns like:
remove_something_from_tablename foo:string bar:integer
create_tablename foo:string bar:integer
create_foo_bar_join_table foo bar
And it then uses a template to generate the according type of migration. If you don't properly pluralize it will be cast into:
remove_containerfrom_create_tasks
Which Rails does not know what to do with as it does not match a known pattern.
Also note despite popular belief migrations are just a DSL to create SQL transformations which is completely unaware about your tables or models. In this case the resulting migration will just blow up when you attempt to run it since you don't have a create_tasks table.
I would roll the missnamed migration back. Delete it then run:
rails g migration RemoveContainerFromTasks container:references
rails db:migrate

Answer 2

Your issue here is that "CreateTasks" is not table in your database. "Tasks" is, however.
rails g migration RemoveContainerFromTasks container:references
will provide you
class RemoveContainerFromTasks < ActiveRecord::Migration[6.1]
def change
remove_reference :tasks, :container, null: false, foreign_key: true
end
end
A migration of this will successfully remove container from your schema.rb file, and subsequently the database system you're using.
Here's some console output, because why not:
unclecid#home:~/Desktop/sample_app$ rails db:migrate
== 20201227150512 CreateTasks: migrating ======================================
-- create_table(:tasks)
-> 0.0022s
== 20201227150512 CreateTasks: migrated (0.0023s) =============================
== 20201227151021 RemoveContainerFromTasks: migrating =========================
-- remove_reference(:tasks, :container, {:null=>false, :foreign_key=>true})
-> 0.0330s
== 20201227151021 RemoveContainerFromTasks: migrated (0.0331s) ================