Migration 执行 SQL 语句
class CreateRecentAnnouncementView < ActiveRecord::Migration def self.up connection.execute("create view recent_announcements as select * from announcements order by id desc limit 10;") end def self.down connection.execute("drop view recent_announcements;") end end
class CreateRecentAnnouncementView < ActiveRecord::Migration def self.up execute <<-SQL CREATE VIEW recent_announcements AS SELECT * FROM announcements ORDER BY id DESC LIMIT 10 SQL end def self.down execute <<-SQL DROP VIEW recent_announcements SQL end end
Rails change column to mediumtext
The reason why the limit values you’re inputting are being ignored is due to how MySQL works. It has four text types, each with their own size limit:
- TINYTEXT - 256 bytes
- TEXT - 65,535 bytes
- MEDIUMTEXT - 16,777,215 bytes
- LONGTEXT - 4,294,967,295 bytes
A text column needs to be one of those four types. You can’t specify a custom length for these types in MySQL.
So if you set a limit on a :text type column, Rails will automatically pick the smallest of those types that can accommodate that value, silently rounding up the limiting value you inputted to one of those four limits above.
t.text :example_text_field, limit: 20 will produce a TINYTEXT field with a limit of 256 bytes, whereas
t.text :example_text_field, limit: 16.megabytes - 1 will produce a MEDIUMTEXT field with a limit of 16,777,215 bytes.
For the shake of the question: “I need to change a column”
change_column :example_table, :example_text_field, :text, limit: 16.megabytes - 1