Khi nào dùng `change` và khi nào cần viết `up`/`down` riêng trong migration?

change dùng cho các operations tự reversible — Rails biết cách đảo ngược:

ruby

def change
  add_column :users, :age, :integer   # down: remove_column
  add_index :users, :email             # down: remove_index
  create_table :posts do |t| ... end  # down: drop_table
end

Một số operations không reversible → phải viết up/down rõ ràng:

ruby

def up
  execute "UPDATE users SET role = 'member' WHERE role IS NULL"
end

def down
  raise ActiveRecord::IrreversibleMigration,
    "Cannot reverse a data-only migration"
end

Operations không reversible: remove_column (mất data type), change_column, execute raw SQL data.

Use change for operations that are inherently reversible — Rails knows how to invert them:

ruby

def change
  add_column :users, :age, :integer   # down: remove_column
  add_index :users, :email             # down: remove_index
  create_table :posts do |t| ... end  # down: drop_table
end

Some operations are not reversible — write explicit up/down:

ruby

def up
  execute "UPDATE users SET role = 'member' WHERE role IS NULL"
end

def down
  raise ActiveRecord::IrreversibleMigration,
    "Cannot reverse a data-only migration"
end

Non-reversible operations: remove_column (loses type info), change_column, raw SQL data mutations.

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Mở danh sách Rails

change dùng cho các operations tự reversible — Rails biết cách đảo ngược:

ruby

def change
  add_column :users, :age, :integer   # down: remove_column
  add_index :users, :email             # down: remove_index
  create_table :posts do |t| ... end  # down: drop_table
end

Một số operations không reversible → phải viết up/down rõ ràng:

ruby

def up
  execute "UPDATE users SET role = 'member' WHERE role IS NULL"
end

def down
  raise ActiveRecord::IrreversibleMigration,
    "Cannot reverse a data-only migration"
end

Operations không reversible: remove_column (mất data type), change_column, execute raw SQL data.

Use change for operations that are inherently reversible — Rails knows how to invert them:

ruby

def change
  add_column :users, :age, :integer   # down: remove_column
  add_index :users, :email             # down: remove_index
  create_table :posts do |t| ... end  # down: drop_table
end

Some operations are not reversible — write explicit up/down:

ruby

def up
  execute "UPDATE users SET role = 'member' WHERE role IS NULL"
end

def down
  raise ActiveRecord::IrreversibleMigration,
    "Cannot reverse a data-only migration"
end

Non-reversible operations: remove_column (loses type info), change_column, raw SQL data mutations.