Assigning default value while creating migration file

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

t.integer :retweets_count, :default => 0

... should work.

See the Rails guide on migrations