Creating a ROM Changeset

Define write rules for a relation. Every data mutation goes through a changeset — never call relation.insert directly.

Quick Reference

• Location: slices/<slice>/changesets/
• Types: ROM::Changeset::Create, ROM::Changeset::Update, ROM::Changeset::Delete.
• Usage: repo.create(attrs) internally calls the changeset.
• Rule: Changesets validate and transform input. Repositories call them.

HARD-GATE

Changeset Constraints:

DO NOT call relation.command(:create).call(...) directly. Use a changeset.
DO NOT put business logic in changesets. Changesets validate and map input only.

Core Process

1. Create the changeset for the target relation:

   module Api
     module Changesets
       class CreateUser < ROM::Changeset::Create
         map do |attrs|
           attrs.merge(
             created_at: Time.now,
             status: "active"
           )
         end
   
         # Validate before persistence
         def default_contract = UserContract.new
       end
     end
   end

2. Run the spec to verify changeset transforms input correctly before wiring into the repository:

   bundle exec rspec spec/changesets/create_user_spec.rb
   # Confirm map block and contract behave as expected

3. Use in the repository — the repository's create method delegates to the changeset:

   class UserRepo < ROM::Repository[:users]
     def create(attrs)
       users.changeset(CreateUser, attrs).commit
     end
   end

4. Update changeset — map input, merge timestamps, validate:

   class UpdateUser < ROM::Changeset::Update
     map do |attrs|
       attrs.merge(updated_at: Time.now)
     end
   
     # Restrict which fields can be updated
     def allowed_keys = [:name, :email, :role]
   end

5. Compose changesets — chain multiple changesets with .data for multi-step writes:

   class UserRepo < ROM::Repository[:users]
     def create_with_profile(user_attrs, profile_attrs)
       user_cs = users.changeset(CreateUser, user_attrs)
       profile_cs = profiles.changeset(CreateProfile, profile_attrs)
   
       # Pass transformed data from the first changeset into the second
       user_cs.data.then { |data| profile_cs.data.merge(owner_id: data[:id]) }
       user_cs.commit
       profile_cs.commit
     end
   end

Extended Resources (Progressive Disclosure)

Load these files only when needed:

• CHANGESET_PATTERNS.md — Patterns: timestamps, allowed keys, custom types, contract integration, testing.

Integration