Trailblazer: Operation

Last updated 13 November 2016 operation v1.1 v2.0

Note: This describes version 1.1.

An operation is a service object.

Operations implement functions of your application, like creating a comment, following a user or exporting a PDF document. Sometimes this is also called command.

Technically, an operation embraces and orchestrates all business logic between the controller dispatch and the persistence layer. This ranges from tasks as finding or creating a model, validating incoming data using a form object to persisting application state using model(s) and dispatching post-processing callbacks or even nested operations.

Note that operation is not a monolithic god object, but a composition of many stakeholders. It is up to you to include features like policies, representers or callbacks.


An operation is to be seen as a function as in Functional Programming. You invoke an operation using the implicit ::call class method.

op = Comment::Create.(comment: {body: "MVC is so 90s."})

This will instantiate the Comment::Create operation for you, run it and return this very instance. The reason the instance is returned is to allow you accessing its contract, validation errors, or other objects you might need for presentation.

Consider this operation instance as a throw-away immutable object. Don’t use it for anything but presentation or you will have unwanted side-effects.

Operation Class

All you need in an operation is a #process method.

class Comment::Create < Trailblazer::Operation
  def process(params)
    puts params

Running this operation will result in the following.

Comment::Create.(comment: {body: "MVC is so 90s."})
#=> {comment: {body: "MVC is so 90s."}}

The params from the invocation get passed into #process.


Normally, operations are working on models. This term does absolutely not limit you to ActiveRecord-style ORM models, though, but can be just anything, for example a OpenStruct composition or a ROM model.

Assigning @model will allow accessing your operation model from the outside.

class Comment::Update < Trailblazer::Operation
  def process(params)
    @model = Comment.find params[:id]

op = Comment::Update.(id: 1)
op.model #=> <Comment id: 1>

Since every public function in your application is implemented as an operation, you don’t access models directly anymore on the outside.


The cool thing about Trailblazer’s operation is that it integrates the validation process using a form object.

This is often done wrong in Rails applications where the controller first instantiates a form and then passes it to a service object. In Trailblazer, the operation is the place for all business logic.

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

  def process(params)
    @model = Comment.new

    validate(params[:comment], @model) do

Using the ::contract block you can define a Reform::Form class that the operation will use for validation (and rendering). Any Reform feature like nesting, populators or complex validations can be used here.

The validate block is only executed when the validation was successful and allows you to save the model and run arbitrary post-processing code. Here, we use the contract’s save which will push the validated data to the model and then save it.

Learn more


Normally, a Create operation will instantiate a new model object, whereas Update, Show, or Delete operations need to find a particular model.

This is such a common workflow for CRUD operations that it is built into Trailblazer in the Operation::Model module.

class Comment::Create < Trailblazer::Operation
  include Model
  model Comment, :create

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

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

Now, the operation takes care of creating the model in validate. Note that there is zero coupling to ActiveRecord: Model will only call Comment.new or Comment.find(id) on the configured model class to accomplish its job, allowing any kind of persistence layer with that API.

Comment::Create.(comment: {body: "MVC is so 90s."}).model #=> <Comment body="MVC ..">

Learn more


There’s two different invocation styles.

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

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

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

The run style returns a result set [result, operation] in both cases.

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.

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

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

This style is often used in framework bindings for Rails, Lotus or Roda when hooking the operation call into the endpoint.

Design Goals

Operations decouple the business logic from the actual framework and from the persistence layer.

This makes it really easy to update or swap the underlying framework or ORM. For instance, operations written in a Rails environment can be run in Sinatra or Lotus as the only coupling happens when querying or writing to the database.

Testing Operations

Operations are incredibly simple to test. All edge-cases can cleanly be tested in unit tests, without the HTTP overhead.

describe Comment::Create do
  it "works" do
    comment = Comment::Create.(comment: {body: "Operation rules!"}).model
    expect(comment.body).to eq("Operation rules!")

Testing With Operations

Another huge advantage is: operations can be used in any environment like scripts, background jobs and will do the exact same as in a controller. This is extremely helpful to use operations as a replacement for test factories.

describe Comment::Update do
  it "updates" do
    comment = Comment::Create.(..) # this is a factory.

    Comment::Update.(id: comment.id, comment: {body: "FTW!"})
    expect(comment.body).to eq("FTW!")

Presenting Operations

The operation is not only helpful for validating and processing data, it can also be used when rendering the form.

operation = Comment::Update.present(id: 1)

Comment::Update will now run the model-finding logic and create the form object for you. It will not run #process.

# Operation finds the model..
operation.model #=> <Comment body="FTW!">
# and provides the Reform object.
@form = operation.contract #=> <Reform::Form ..>

As Reform works with most form builders out-of-the-box, you can pass the form right into it.

= simple_form_for @form do |f|
  = f.input :body
  = f.button :submit

This normally covers the logic for two controller actions, e.g. new and create.


Operation has many optional features like authorization, callbacks, polymorphic builders, etc.