What's the syntax for dropping a data开发者_Go百科base table column through a Rails migration?

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remove_column :table_name, :column_name

For instance:

remove_column :users, :hobby

would remove the hobby Column from the users table.

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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

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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.

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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

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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

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Clear & Simple Instructions for Rails 5 & 6

• WARNING: You will lose data.
• Warning: the below instructions are for trivial migrations. For complex migrations with e.g. millions of rows, read/write dbs, clusters, this advice is not for you:

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1. Create a migration

Run the following command in your terminal:

rails generate migration remove_fieldname_from_tablename fieldname:fieldtype (Table name in plural, as per convention. See the documentation here. )

Example: rails g migration RemoveAcceptedFromQuotes accepted:boolean

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!

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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.

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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

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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.

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You can try the following:

remove_column :table_name, :column_name

(Official documentation)

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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.

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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

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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

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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

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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

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Heres one more from rails console

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

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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

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Simply, You can remove column

remove_column :table_name, :column_name

For Example,

remove_column :posts, :comment

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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

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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

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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

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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

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you can use rails migration command

rails generate migration RemoveColumnNameFromTableName column_name:column_type

than you can migrate the database:

rails db:migrate

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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”)