For older versions of Rails

ruby script/generate migration RemoveFieldNameFromTableName field_name:datatype

For Rails 3 and up

rails generate migration RemoveFieldNameFromTableName field_name:datatype

Rails 4 has been updated, so the change method can be used in the migration to drop a column and the migration will successfully rollback. Please read the following warning for Rails 3 applications:

Rails 3 Warning

Please note that when you use this command:

rails generate migration RemoveFieldNameFromTableName field_name:datatype

The generated migration will look something like this:

  def up
    remove_column :table_name, :field_name
  end

  def down
    add_column :table_name, :field_name, :datatype
  end

Make sure to not use the change method when removing columns from a database table (example of what you don’t want in the migration file in Rails 3 apps):

  def change
    remove_column :table_name, :field_name
  end

The change method in Rails 3 is not smart when it comes to remove_column, so you will not be able to rollback this migration.

In a rails4 app it is possible to use the change method also for removing columns. The third param is the data_type and in the optional forth you can give options. It is a bit hidden in the section ‘Available transformations’ on the documentation .

class RemoveFieldFromTableName < ActiveRecord::Migration
  def change
    remove_column :table_name, :field_name, :data_type, {}
  end
end

There are two good ways to do this:

remove_column

You can simply use remove_column, like so:

remove_column :users, :first_name

This is fine if you only need to make a single change to your schema.

change_table block

You can also do this using a change_table block, like so:

change_table :users do |t|
  t.remove :first_name
end

I prefer this as I find it more legible, and you can make several changes at once.

Here’s the full list of supported change_table methods:

http://apidock.com/rails/ActiveRecord/ConnectionAdapters/SchemaStatements/change_table

Clear & Simple Instructions for Rails 5 & 6

• WARNING: You will lose data if you remove a column from your database. To proceed, see below:
• Warning: the below instructions are for trivial migrations. For complex migrations with e.g. millions and millions of rows, you will have to account for the possibility of failures, you will also have to think about how to optimise your migrations so that they run swiftly, and the possibility that users will use your app while the migration process is occurring. If you have multiple databases, or if anything is remotely complicated, then don’t blame me if anything goes wrong!

1. Create a migration

Run the following command in your terminal:

rails generate migration remove_fieldname_from_tablename fieldname:fieldtype

Note: the table name should be in plural form as per rails convention.

Example:

In my case I want to remove the accepted column (a boolean value) from the quotes table:

rails g migration RemoveAcceptedFromQuotes accepted:boolean

See the documentation re: a convention when adding/removing fields to a table:

There is a special syntactic shortcut to generate migrations that add
fields to a table.

rails generate migration add_fieldname_to_tablename fieldname:fieldtype

2. Check the migration

# db/migrate/20190122035000_remove_accepted_from_quotes.rb
class RemoveAcceptedFromQuotes < ActiveRecord::Migration[5.2]
  # with rails 5.2 you don't need to add a separate "up" and "down" method.
  def change
    remove_column :quotes, :accepted, :boolean
  end
end

3. Run the migration

rake db:migrate or rails db:migrate (they’re both the same)

….And then you’re off to the races!

Generate a migration to remove a column such that if it is migrated (rake db:migrate), it should drop the column. And it should add column back if this migration is rollbacked (rake db:rollback).

The syntax:

remove_column :table_name, :column_name, :type

Removes column, also adds column back if migration is rollbacked.

Example:

remove_column :users, :last_name, :string

Note: If you skip the data_type, the migration will remove the column successfully but if you rollback the migration it will throw an error.

in rails 5 you can use this command in the terminal:

rails generate migration remove_COLUMNNAME_from_TABLENAME COLUMNNAME:DATATYPE

for example to remove the column access_level(string) from table users:

rails generate migration remove_access_level_from_users access_level:string

and then run:

rake db:migrate

Remove Columns For RAILS 5 App

rails g migration Remove<Anything>From<TableName> [columnName:type]

Command above generate a migration file inside db/migrate directory. Snippet blow is one of remove column from table example generated by Rails generator,

class RemoveAgeFromUsers < ActiveRecord::Migration
  def up
    remove_column :users, :age
  end
  def down
    add_column :users, :age, :integer
  end
end

I also made a quick reference guide for Rails which can be found at here.

You can try the following:

remove_column :table_name, :column_name

(Official documentation)

rails g migration RemoveXColumnFromY column_name:data_type

X = column name
Y = table name

EDIT

Changed RemoveXColumnToY to RemoveXColumnFromY as per comments – provides more clarity for what the migration is actually doing.

To remove the column from table you have to run following migration:

rails g migration remove_column_name_from_table_name column_name:data_type

Then run command:

rake db:migrate

Give below command it will add in migration file on its own

rails g migration RemoveColumnFromModel

After running above command you can check migration file remove_column code must be added there on its own

Then migrate the db

rake db:migrate

remove_column in change method will help you to delete the column from the table.

class RemoveColumn < ActiveRecord::Migration
  def change
    remove_column :table_name, :column_name, :data_type
  end
end

Go on this link for complete reference : http://guides.rubyonrails.org/active_record_migrations.html

For removing column from table in just easy 3 steps as follows:

1. write this command

rails g migration remove_column_from_table_name

after running this command in terminal one file created by this name and time stamp (remove_column from_table_name).

Then go to this file.

2. inside file you have to write

   remove_column :table_name, :column_name

3. Finally go to the console and then do

   rake db:migrate

Heres one more from rails console

ActiveRecord::Migration.remove_column(:table_name, :column_name)

Simply, You can remove column

remove_column :table_name, :column_name

For Example,

remove_column :posts, :comment

first try to create a migration file running the command:

rails g migration RemoveAgeFromUsers age:string

and then on the root directory of the project run the migration running the command:

rails db:migrate

Through
remove_column :table_name, :column_name
in a migration file

You can remove a column directly in a rails console by typing:
ActiveRecord::Base.remove_column :table_name, :column_name

Do like this;

rails g migration RemoveColumnNameFromTables column_name:type

I.e. rails g migration RemoveTitleFromPosts title:string

Anyway, Would be better to consider about downtime as well since the ActiveRecord caches database columns at runtime so if you drop a column, it might cause exceptions until your app reboots.

Ref: Strong migration

1. Mark the column as ignored in the model

class MyModel < ApplicationRecord
  self.ignored_columns = ["my_field"]
end

2. Generate a migration

$ bin/rails g migration DropMyFieldFromMyModel

3. Edit the migration

class DropMyFieldFromMyModel < ActiveRecord::Migration[6.1]
  def change
    safety_assured { remove_column :my_table, :my_field }
  end
end

4. Run the migration

$ bin/rails db:migrate

Step 1: Create a migration

  rails g migration remove_column_name_from_table

Step 2: Change code in file migration just created

rails version < 3

  def change
    remove_column :table_name, :column_name, :datatype
  end

rails version >= 3

  def change
    remove_column :table_name, :column_name
  end

Step 3: Migrate

rake db:migrate

Just run this in the rails console

ActiveRecord::Base.connection.remove_column("table_name", :column_name, :its_data_type)

or

TableName.find_by_sql(“ALTER TABLE table_name DROP column_name”)