Deleting all data from a table in Ruby on Rails

If you need to refresh your database, you might find that you need to delete every row from a specific table during development.

That is slow 🐢. Freaking slow 🐄.

Glad Rails have many small tricks to solve everything 🌈 (but still not documented properly 😂).

There are several ways to achieve this. I will rank it from the slowest/scary/bad to the simplest and yet powerful solution.

Let's go !!!

3) Drop the whole database and bring it back up again

We can achieve this with rake:db:drop, then rake:db:setup.

Wuuu, drop the database then create a new one, load the schema, and initialize it with the seed data.

Slow & Expensive !!! 🐢

2) destroy_all

Destroys the records by instantiating each record and calling its #destroy method. Each object's callbacks are executed (including :dependent association options).

Records are instantiated and it invokes before_remove, after_remove , before_destroy and after_destroy callbacks.

Examples

# 1
Person.where(sex: "male").destroy_all # 😂

# 2
class Author < ActiveRecord::Base
  has_many :books
end

author.books.size # => 3
author.books
# => [
#       #<Book id: 1, name: "Sapiens", author_id: 1>,
#       #<Book id: 2, name: "The Artist's Way", author_id: 2>,
#       #<Book id: 3, name: "Remote", author_id: 3>
#    ]

author.books.destroy_all

author.books.size # => 0
author.books      # => []

Book.find(1) # => Couldn't find Book with id=1

Note: Instantiation, callback execution, and deletion of each record can be time consuming when you're removing many records at once. It generates at least one SQL DELETE query per record (or possibly more, to enforce your callbacks).

1) delete_all 👑

If you want to delete many rows quickly, without concern for their associations or callbacks, use delete_all instead.

Boot up a console and call delete_all on your model:

% rails c
> Book.count
# => 1200 
> Book.delete_all
# => 1200
> Book.count
# => 0

Deletes the records without instantiating the records first, and hence not calling the #destroy method nor invoking callbacks.

This is a single SQL DELETE statement that goes straight to the database, much more efficient than destroy_all.

You need to be careful on two things:

1- As .delete_all method does not instantiate any object hence does not provide any callback (before_* and after_destroy don't get triggered).

2- Be careful with relations, in particular :dependent rules defined on associations are not honored. Returns the number of rows affected.

Post.where(person_id: 5).where(category: ['Something', 'Else']).delete_all

Both calls delete the affected posts all at once with a single DELETE statement.

If you need to destroy dependent associations or call your before_* or after_destroy callbacks, use the destroy_all method instead.

More on here

The end

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