migration integer limit option

https://gist.github.com/stream7/1069589

:limit	    Numeric Type	Column Size	Max value
1	     tinyint	         1 byte	          127
2	     smallint	         2 bytes	32767
3	     mediumint	         3 byte	        8388607
nil, 4, 11    int(11)	         4 byte	       2147483647
5..8	      bigint	         8 byte	     9223372036854775807

关于sequence 字段
limit: 2 是两个字节，16 位，2 的 15 次方是 32767

class CreateTableSubjections < ActiveRecord::Migration[5.0]
  def change
    create_table :table_subjections do |t| 
      t.belongs_to :subject, index: true
      t.belongs_to :video, index: true
      t.string :title, limit: 40
      t.string :sub_title, limit: 40
      t.string :description, limit: 500
      t.integer :status, limit: 1
      t.integer :sequence, limit: 2

      t.timestamps
    end 
  end 
end

对于多态关联的表

class CreateSubjections < ActiveRecord::Migration[5.0]
  def change
    create_table :subjections do |t| 
      t.belongs_to :subject, index: true
      t.string :title, limit: 40
      t.string :sub_title, limit: 40
      t.string :description, limit: 500 
      t.string :subjectable_type, limit: 20
      t.integer :subjectable_id
      t.integer :status, limit: 1
      t.integer :sequence, limit: 2

      t.timestamps
    end 

    add_index :subjections, [:subjectable_type, :subjectable_id]
  end 
end

t.integer :subjectable_id

这个字段不需要设置limit，因为这个字段是用来做外键的，存储的是数据库的自增主键。