Operation API

Last updated 20 December 2016 trailblazer v1.1 v2.0

This document discusses the callstack from top to bottom.

Call Style

The call style returns the operation when the validation was successful. With invalid data, it will raise an InvalidContract exception.

Comment::Create.(comment: {body: "MVC's so 90s."}) #=> <Comment::Create @model=..>
Comment::Create.(comment: {}) #=> exception raised!

The call style is popular for tests and on the console.

Run Style

The run style returns a result set [result, operation] for both valid and invalid invocation.

res, operation = Comment::Create.run(comment: {body: "MVC is so 90s."})

However, it also accepts a block that’s run in case of a successful validation. When run with block, only the operation instance is returned as the block represents a valid state.

operation = Comment::Create.run(comment: {}) do |op|
  puts "Hey, #{op.model} was created!" and return # not run.

puts "That's wrong: #{operation.errors}"

To conveniently handle the inverse case where the block should be run in case of an invalid operation, use ::reject.

res, operation = Comment::Create.reject(comment: {}) do |op|
  puts "this is all wrong! #{operation.errors}"

Regardless of the style, you always get the operation instance. This is only for presentation. Please treat it as immutuable.

Operations in Tests

Operations when used test factories are usually invoked with the call style.

let(:comment) { Comment::Create.(valid_comment_params).model }

Using operations as test factories is a fundamental concept of Trailblazer to remove buggy redundancy in tests and manual factories. Note that you might use FactoryGirl to create params hashes.


Here’s the default call stack of methods involved when running an Operation.

├── ::build_operation
│   ├── #initialize
│   │   ├── #setup!
│   │   │   ├── #assign_params!
│   │   │   │   │   ├── #params!
│   │   │   ├── #setup_params!
│   │   │   ├── #build_model!
│   │   │   │   ├── #assign_model!
│   │   │   │   │   ├── #model!
│   │   │   │   ├── #setup_model!
│   ├── #run
│   │   ├── #process
  1. In case of a polymorphic setup when you want different operation classes to handle different contexts, configured builders will be run by ::build_operation to figure out the class to instantiate.
  2. Override #setup_params! to normalize the incoming parameters. This implies that you have to change the original hash.
  3. Override #params! if you want to use a different params hash - which you simply return from this method. This allows to keep the original params immutable.
  4. Override #model! to compute the operation’s model.
  5. Override #setup_model! for tasks such as adding nested models to the operation’s model.
  6. Implement #process for your business logic.

The Operation::Model module to create/find models automatically hooks into those methods.


The #process method is the pivot of any operation. Here, business logic and validations get executed and dispatched.

Its only argument is the params hash being passed into Op.({..}). Note that you don’t even have to use a contract or a model.

class Comment::Create < Trailblazer::Operation
  def process(params)
    # do whatever you feel like.


The validate method will instantiate the operation’s Reform form with the model and validate it. If you pass a block to it, this will be executed only if the validation was successful (valid).

class Comment::Update < Trailblazer::Operation
  contract do
    property :body, validates: {presence: true}

  def process(params)
    manual_model = Comment.find(1)

    validate(params[:comment], manual_model) do

Note that when Model is not included, you have to pass in the model as the second argument to #validate.

However, since most operations use Model, we can omit a lot of code here.

class Comment::Update < Trailblazer::Operation
  include Model
  model Comment, :update

  contract do
    property :body, validates: {presence: true}

  def process(params)
    validate(params[:comment]) do

With Model included, #validate only takes one argument: the params to validate.

Validate Internals

Internally, this is what happens.

def validate(params)
  @contract = self.class.contract_class.new(model)
  @valid = @contract.validate(params)
  1. The contract is instantiated using the operation’s #model. The contract is available via #contract throughout the operation (even before you call #validate, which will use the same instance).
  2. It then validates the incoming params, assigns values and errors on the contract object and returns the result.

Validate: Handling Invalid

You don’t have to use the block-style #validate. It returns the validation result and allows an if/else, too.

def process(params)
  if validate(params[:comment])
    contract.save # success.
    notify! # invalid.


Normally, the contract is instantiated when calling validate. However, you can access the contract before that. The contract is memoized and validate will use the existing instance.

def process(params)
  contract.body = "Static"
  validate(params[:comment]) do # will use above contract.
    contract.save # also the same as above.

The #contract method always returns the Reform object. It has the identical API and allows to sync, save, etc.

This is not only useful in the validate block, but also afterwards, for example to render the invalid form.

operation = Comment::Create.(params)
form      = operation.contract

Note that you don’t have to run an operation in order to get its form object (which would invoke the #process method). You can use ::present instead.

Validation Errors

You can access the contracts Errors object via Operation#errors.


To grab the operation’s form object for presentation without running process, use ::present.

op = Comment::Create.present(params)
op.model    #=> model is available!
op.contract #=> form object, too.

In the callstack, this simply runs #initialize, only.

This is used when presenting the operation’s form or model, for example in new, edit or show actions in a controller.

Composable Interface: Contract

The operation’s contract is just a plain Reform class and doesn’t know anything about the composing operation.

This is why you may reference arbitrary contract classes using ::contract. That’s helpful if you keep contracts in separate files, or reuse them without inheriting from another operation.

class Comment::Delete < Trailblazer::Operation
  contract CommentForm # a plain Reform::Form class.

You can also reference a contract from another operation.

class Comment::Delete < Trailblazer::Operation
  contract Update.contract

Note that ::contract will subclass the referenced contract class, making it a copy of the original, allowing you to add and remove fields and validations in the copy.

You can also copy and refine the contract.

class Comment::Delete < Trailblazer::Operation
  contract Update.contract do
    property :upvotes

To reference without copying, use Operation::contract_class=(constant)

Marking Operation as Invalid

Sometimes you don’t need a form object but still want the validity behavior of an operation.

def process(params)
  return invalid! unless params[:id]


ActiveModel Semantics

When using Reform::Form::ActiveModel (which is used automatically in a Rails environment to make form builders work) you need to invoke model Comment in the contract. This can be inferred automatically from the operation by including Model::ActiveModel.

class Create < Trailblazer::Operation
  include Model
  include Model::ActiveModel

  model Comment

  contract do # no need to call ::model, here.
    property :text

If you want that in all CRUD operations, check out how you can include it automatically.