How to write a migration from Rails datetime type to date type

** "It looks good with date instead of this column datetime" **

I used datetime because I deal with dates, but when I think about it carefully, I don't need" time ", so I want to convert it to date.

This goal

--Book models have a release date released_at (datetime) --I want to migrate and make it released_on (date)


Datetime → date conversion using change

def change
  rename_column :books, :released_at, :released_on
  change_column :books, :released_on, 'date USING CAST(released_on AS date)'

Can you db: rollback?

That's okay, just wait a minute with git commi .., **! ** **

Keep in mind that migrations roll back.

Let's see if we can roll back the migration mentioned above.

$ rails db:rollback

== ..... : reverting ========
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:

This migration uses change_column, which is not automatically reversible.
To make the migration reversible you can either:
1. Define #up and #down methods in place of the #change method.
2. Use the #reversible method to define reversible behavior.

What caused the error

--When you roll back with change_column, you can't restore it because you don't know the previous type information.

I was able to convert the type with 'date USING CAST (released_on AS date)', but the rollback failed because ** "type information before conversion" ** was missing.

As one way to solve this, let's rewrite it as ** up / down method **.

Method using up / down

def up
  rename_column :books, :published_at, :release_on
  change_column :books, :release_on, 'date USING CAST(release_on AS date)'

def down
  rename_column :books, :release_on, :published_at
  change_column :books, :published_at, 'timestamp USING CAST(published_at AS timestamp)'

By doing this

--db: migrate executes datetime-> date of ʻup --db: rollback executes down date-> datetime`

Bonus: datetime USING CAST gives an error in PostgreSQL

change_column :books, :released_at, 'datetime USING CAST(released_at AS datetime)'
PG::UndefinedObject: ERROR:  type "datetime" does not exist
LINE 1: ...released_at" TYPE datetime USING CAST(published_at AS datetime)

It was said that datetime in the Rails world becomestimestamp in the PostgreSQL world: sweat_smile:

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