Migrations

Question 1

How to create a simple migration?

Answer 1

rails g migration sample

Question 2

How to generate a migration with a table creation?

Answer 2

# create and table name are keywords there
rails g migration create_table_name

creates

def change
create_table :table_names do |t|
end
end

Question 3

How to generate a migration to create table with a column field?

Answer 3

rails g migration create_table_names name:string

creates

  def change
    create_table :table_names do |t|
      t.string :name
    end
  end

Question 4

How to generate a migration to remove a column?

Answer 4

# you need to use Remove and From 
rails g migration RemoveColumnFromTableName sample

creates

def change
remove_column :table_names, :sample, :string
end

Question 5

How to generate a new column with an index?

Answer 5

rails g migration AddSampleToTableName sample:string:index

Question 6

How do rails know which migration should be run or not?

Answer 6

Rails create a row with a timestamp in schema_migrations table after each running migration.

Question 7

How to run and rollback migration?

Answer 7

rails db:migrate

rails db:rollback

Question 8

How to use up/down inside migrations?

Answer 8

# up runs on db:migrate
# down runs on db:rollback

Question 9

How to create a table inside a migration?

Answer 9

create_table :hello

Question 10

How to create a table with a field?

Answer 10

create_table :hello_world do |t|
t.column :sample, :string
end

Question 11

How to create a table with a field?

Answer 11

create_table :hello_world do |t|
t.column :sample, :string
end

Question 12

What are the differences between :decimal and :float?

Answer 12

:decimal - Stored with specified precision. Use for math that requires accuracy.

:float - Floating-point decimal number with fixed precision depending on platform. Do not use for math that requires accuracy due to rounding errors.

Question 13

How to create a table with a field and index?

Answer 13

create_table :hello_world do |t|
t.column :sample, :string
t.index :sample
end

Question 14

How to add belongs_to association on migration?

Answer 14

# references
    create_table :hello_world do |t|
      t.references :dog
    end
# belongs_to
    create_table :hello_world do |t|
      t.belongs_to :dog
    end
# add column with index 
    create_table :hello_world do |t|
      t.column :dog_id, :integer
      t.index :dog_id
    end

Question 15

How to add belongs_to association on migration? ( 3 ways)

Answer 15

# add column with index 
    create_table :hello_world do |t|
      t.column :dog_id, :integer
      t.index :dog_id
    end
# references
    create_table :hello_world do |t|
      t.references :dog
    end
# belongs_to
    create_table :hello_world do |t|
      t.belongs_to :dog
    end

Question 16

How to add a new column to the table?

Answer 16

def change
    add_column :table_names, :my_column, :integer
  end

Question 17

How to add additional fields to the new column? (default, null, limit, index)

Answer 17

def change
    add_column :table_names, :my_column, :integer, default: 1, limit: 10, index: true, null: false
  end

Question 18

How to show actual status of migration?

Answer 18

rake db:migrate:status

Question 19

What are the differences between db:migrate and db:schema:load?

Answer 19

• rake db:migrate runs migrations that have not run yet
• rake db:schema:load loads the schema.db file into database.

Advice: when you are adding a new migration to an existing app then you need to run rake db:migrate, but when you join to existing application (especially some old application), or when you drop your applications database and you need to create it again, always run rake db:schema:load to load schema

Question 20

How to remove a column from the table?

Answer 20

def change
remove_column :cats, :name
end

Question 21

How to drop a table?

Answer 21

def change
drop_table :cats
end

Question 22

How to change column type?

Answer 22

def change
    change_column :dogs, :name, :text
  end

Question 23

How to rename column?

Answer 23

def change
    rename_column :dogs, :name, :name2
  end

Question 24

How to add polymorphic migration? And what field this connection generates?

Answer 24

create_table :cats do |t|
t.references :owner, polymorphic: true
end

# owner_id
# owner_type

Question 25

How to create a migration to create a join table for 2 tables?

Answer 25

def change
    create_join_table :dogs, :users do |t|
      t.index [:dog_id, :user_id]
      t.index [:user_id, :dog_id]
    end
  end

the same

  def change
    create_table :dogs_users do |t|
      t.belongs_to :user
      t.belongs_to :dog
    end
  end

# creates the following
Table - dogs_users ( alphabetic order )
Fields - dog_id, user_id
Indexes

Question 26

Is the version number in ‘schema.rb’ used for anything? Should we carry about it?

Answer 26

RAILS AUTOMATICLY SET THE LATEST VERSION OF MIGRATION after rails db:migrate to schema.rb.

The real effect of the migration version is that all prior migrations (as found in the db/migrate directory) are assumed to have been run.

1) Shema version 2020, old migration 2019 what will happen after rake db:migrate?

Migration will be run, shema version has the latest migration version ( 2020 )

2) Shema version 2020, old migration 2019 what will happen after rake db:schema:load?

Migration won’t be added to the schema.rb. And db:migrate will not run this migration anymore ( schema_migration table already contains all migrations). If migration version will be after scheme version db:migrate will work ok.

3) Shema version 2020, new migration 2021 (but alredy in schema ) what will happen after rake db:schema:load and db:migrate?
- Error that this table already exist
- The problem occurs if someone creates a migration with a timestamp prior to your schema.rb’s :version. If you db:migrate, you’ll apply their migration, your schema.rb will be updated (but retain the same, higher :version), and everything is fine. But if you should happen to db:schema:load (or db:reset) instead, you’ll not only be missing their migration, butassume_migrated_upto_version will mark their migration as having been applied.

Question 27

How to add index/column directly in rails c ?

Answer 27

ActiveRecord::Migration.add_index :ahoy_messages, :mailer

or

ActiveRecord::Migration.remove_index :ahoy_messages, :mailer

Question 28

How to check all available indexes on the table?

Answer 28

ActiveRecord::Base.connection.indexes(:ahoy_messages)