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)