Version 3.0

Repositories » Changesets

You already learned how to simply use commands with repositories in the Quick Start section, but there's so much more to it! In this section you're going to learn how to use Changesets along with repositories.

Persisting data in a database can be complex. You often need to pre-process the data before it can be persisted. Whenever additional processing needs to happen you can use Changesets to prepare the data before they can be sent down to the command.

Changesets can be build using Repository#changeset method, here's how you can work with them:

require 'rom-repository'

rom = ROM.container(:sql, 'sqlite::memory') do |config|
  config.default.create_table(:users) do
    primary_key :id
    column :name, String, null: false
    column :email, String, null: false

class UserRepo < ROM::Repository[:users]
  commands :create, update: :by_pk

user_repo =

user = user_repo.create(name: 'Jane', email: '')

changeset = user_repo.changeset(, name: 'Jane Doe')

changeset.diff? # true
changeset.diff # {:name=>"Jane Doe"}

Changesets are compatible with commands, as they implement Hash interface, this means you can pass them to commands. In order to save a changeset, simply pass it to update command method:

user_repo.update(, changeset)
# => #<ROM::Struct[User] id=1 name="Jane Doe" email="">

Repositories will not execute an update command if there's no diff between the original tuple and the new one, i.e.:

changeset = user_repo.changeset(, email: '')

changeset.diff? # false

# no UPDATE query is executed
user_repo.update(, changeset)

Changeset Mapping

Changeset has an extendible data-pipe mechanism available via (for preconfigured mapping) and Changeset#map (for on-demand run-time mapping).

Changeset mappings support all transformation functions from transproc project, and in addition to that we have:

  • :add_timestamps–sets created_at and updated_at timestamps (don't forget to add those fields to the table in case of using rom-sql)
  • :touch–sets updated_at timestamp

You can override the timestamps by simply setting them in the input data.

Pre-configured mapping

If you want to process data before sending them to be persisted, you can define a custom Changeset class and specify your own mapping. Let's say we have a nested hash with address key but we store it as a flat structure with address attributes having address_* prefix:

class NewUserChangeset < ROM::Changeset::Create
  map do
    unwrap :address, prefix: true

Then we can ask the user repo for our changeset:

user_data = { name: 'Jane', address: { city: 'NYC', street: 'Street 1' } }

changeset = user_repo.changeset(NewUserChangeset).data(user_data)

# { name: 'Jane', address_city: 'NYC', address_street: 'Street 1' }


Custom mapping block

If you don't want to use built-in transformations, simply configure a mapping and pass tuple argument to the map block:

class NewUserChangeset < ROM::Changeset::Create
  map do |tuple|

user_data = { name: 'Jane' }

changeset = user_repo.changeset(NewUserChangeset).data(user_data)

# { name: 'Jane', created_on: <Date: 2017-01-21 ((2457775j,0s,0n),+0s,2299161j)> }

# => #<ROM::Struct[User] id=1 name="Jane" created_on=2017-01-21>

Custom mapping blocks are executed in the context of your changeset objects, which means you have access to changeset's state

On-demand mapping

There are situations where you would like to perform an additional mapping but adding a special changeset class would be an overkill. That's why it's possible to apply additional mappings at run-time without having to use a custom changeset class. To do this simply use Changeset#map method:

changeset = user_repo
  .changeset(name: 'Joe', email: '')

# => #<ROM::Struct[User] id=1 name="Joe" email="" created_at=2016-07-22 14:45:02 +0200 updated_at=2016-07-22 14:45:02 +0200>

Committing changesets

Changesets can be committed without the need to use repository command methods. The difference is that by committing a changeset you get back raw data returned from your database:

new_user = repo.changeset(name: 'Jane')
=> #<ROM::Changeset::Create relation=ROM::Relation::Name(users) data={:name=>"Jane"}>

=> #<ROM::Struct[User] id=3 name="Jane">

=> {:id=>4, :name=>"Jane"}

Learn more

Check out API docs:


Now you can learn how to define custom changeset classes.