Ruby-on-rails – Assigning default value while creating migration file

migrationruby-on-rails

rails generate migration AddRetweetsCountToTweet retweets_count:integer

Ok I use above line to create migration file that automatically generates code in the generated file to add a column to a model Tweet with datatype integer. Now I want to add default value to the added column while generating the migration file. Is that possible? I googled it but couldn't find. Guys need help.

Best Answer

Default migration generator does not handle default values (column modifiers are supported but do not include default or null), but you could create your own generator.

You can also manually update the migration file prior to running rake db:migrate by adding the options to add_column:

add_column :tweet, :retweets_count, :integer, :null => false, :default => 0

... and read Rails API

Related Solutions

Ruby-on-rails – Run a single migration file

Assuming fairly recent version of Rails you can always run:

rake db:migrate:up VERSION=20090408054532

Where version is the timestamp in the filename of the migration.

Edit: At some point over the last 8 years (I'm not sure what version) Rails added checks that prevent this from running if it has already been run. This is indicated by an entry in the schema_migrations table. To re-run it, simply execute rake db:migrate:redo VERSION=20090408054532 instead.

Ruby-on-rails – Using Rails, how can I set the primary key to not be an integer-typed column

Unfortunately, I've determined it's not possible to do it without using execute.

Why it doesn't work

By examining the ActiveRecord source, we can find the code for create_table:

In schema_statements.rb:

def create_table(table_name, options={})
  ...
  table_definition.primary_key(options[:primary_key] || Base.get_primary_key(table_name.to_s.singularize)) unless options[:id] == false
  ...
end

So we can see that when you try to specify a primary key in the create_table options, it creates a primary key with that specified name (or, if none is specified, id). It does this by calling the same method you can use inside a table definition block: primary_key.

In schema_statements.rb:

def primary_key(name)
  column(name, :primary_key)
end

This just creates a column with the specified name of type :primary_key. This is set to the following in the standard database adapters:

PostgreSQL: "serial primary key"
MySQL: "int(11) DEFAULT NULL auto_increment PRIMARY KEY"
SQLite: "INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL"

The workaround

Since we're stuck with these as the primary key types, we have to use execute to create a primary key that is not an integer (PostgreSQL's serial is an integer using a sequence):

create_table :employees, {:id => false} do |t|
  t.string :emp_id
  t.string :first_name
  t.string :last_name
end
execute "ALTER TABLE employees ADD PRIMARY KEY (emp_id);"

And as Sean McCleary mentioned, your ActiveRecord model should set the primary key using set_primary_key:

class Employee < ActiveRecord::Base
  set_primary_key :emp_id
  ...
end