A helper for creating declarative interfaces in controllers
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Latest commit a4ba6bd Oct 31, 2016 Matt Polito and Vinicius Ferreira Negrisolo Bump to v3.0.2


Decent Exposure

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DecentExposure 3.0 is a major change from < 3.0.

Version 3.0 will support Rails 4 and 5.

Be aware... mild API changes ahead. Check this API changes in version 3


Add this line to your application's Gemfile:

gem 'decent_exposure', '3.0.0'

And then execute:

$ bundle

Or install it yourself as:

$ gem install decent_exposure


The whole API consists of three methods so far: expose, expose!, and exposure_config.

In the simplest scenario you'll just use it to expose a model in the controller:

class ThingsController < ApplicationController
  expose :thing

Now every time you call thing in your controller or view, it will look for an ID and try to perform Thing.find(id). If the ID isn't found, it will call Thing.new(things_params). The result will be memoized in an @exposed_thing instance variable.

Example Controller

Here's what a standard Rails 5 CRUD controller using Decent Exposure might look like:

class ThingsController < ApplicationController
  expose :things, ->{ Thing.all }
  expose :thing

  def create
    if thing.save
      redirect_to thing_path(thing)
      render :new

  def update
    if thing.update(thing_params)
      redirect_to thing_path(thing)
      render :edit

  def destroy
    redirect_to things_path


  def thing_params
    params.require(:thing).permit(:foo, :bar)

Under the Hood

The default resolving workflow is pretty powerful and customizable. It could be expressed with the following pseudocode:

def fetch(scope, id)
  instance = id ? find(id, scope) : build(build_params, scope)

def id
  params[:thing_id] || params[:id]

def find(id, scope)

def build(params, scope)
  scope.new(params) # Thing.new(params)

def scope
  model # Thing

def model
  exposure_name.classify.constantize # :thing -> Thing

def build_params
  if respond_to?(:thing_params, true) && !request.get?

def decorate(thing)

The exposure is also lazy, which means that it won't do anything until you call the method. To eliminate this laziness you can use the expose! macro instead, which will try to resolve the exposure in a before filter.

It is possible to override each step with options. The acceptable options to the expose macro are:


This is the entry point. The fetch proc defines how to resolve your exposure in the first place.

expose :thing, fetch: ->{ get_thing_some_way_or_another }

Because the above behavior overrides the normal workflow, all other options would be ignored. However, Decent Exposure is decent enough to actually blow up with an error so you don't accidentally do this.

There are other less verbose ways to pass the fetch block, since you'll probably be using it often:

expose(:thing){ get_thing_some_way_or_another }


expose :thing, ->{ get_thing_some_way_or_another }

Or even shorter

expose :thing, :get_thing_some_way_or_another

There is another shortcut that allows you to redefine the entire fetch block with less code:

expose :comments, from: :post
# equivalent to 
expose :comments, ->{ post.comments }


The default fetch logic relies on the presence of an ID. And of course Decent Exposure allows you to specify how exactly you want the ID to be extracted.

Default behavior could be expressed using following code:

expose :thing, id: ->{ params[:thing_id] || params[:id] }

But nothing is stopping you from throwing in any arbitrary code:

expose :thing, id: ->{ 42 }

Passing lambdas might not always be fun, so here are a couple of shortcuts that could help make life easier.

expose :thing, id: :custom_thing_id
# equivalent to
expose :thing, id: ->{ params[:custom_thing_id] }

expose :thing, id: [:try_this_id, :or_maybe_that_id]
# equivalent to
expose :thing, id: ->{ params[:try_this_id] || params[:or_maybe_that_id] }


If an ID was provided, Decent Exposure will try to find the model using it. Default behavior could be expressed with this configuration:

expose :thing, find: ->(id, scope){ scope.find(id) }

Where scope is a model scope, like Thing or User.active or Post.published.

Now, if you're using FriendlyId or Stringex or something similar, you'd have to customize your finding logic. Your code might look somewhat like this:

expose :thing, find: ->(id, scope){ scope.find_by!(slug: id) }

Again, because this is likely to happen a lot, Decent Exposure gives you a decent shortcut so you can get more done by typing less.

expose :thing, find_by: :slug


When an ID is not present, Decent Exposure tries to build an object for you. By default, it behaves like this:

expose :thing, build: ->(thing_params, scope){ scope.new(thing_params) }


These options are responsible for calulating params before passing them to the build step. The default behavior was modeled with Strong Parameters in mind and is somewhat smart: it calls the thing_params controller method if it's available and the request method is not GET. In all other cases it produces an empty hash.

You can easily specify which controller method you want it to call instead of thing_params, or just provide your own logic:

expose :thing, build_params: :custom_thing_params
expose :other_thing, build_params: ->{ { foo: "bar" } }


def custom_thing_params
  # strong parameters stuff goes here


Defines the scope that's used in find and build steps.

expose :thing, scope: ->{ current_user.things }
expose :user, scope: ->{ User.active }
expose :post, scope: ->{ Post.published }

Like before, shortcuts are there to make you happier:

expose :post, scope: :published
# equivalent to
expose :post, scope: ->{ Post.published }


expose :thing, parent: :current_user
# equivalent to:
expose :thing, scope: ->{ current_user.things }


Allows you to specify the model class to use. Pretty straightforward.

expose :thing, model: ->{ AnotherThing }
expose :thing, model: AnotherThing
expose :thing, model: "AnotherThing"
expose :thing, model: :another_thing


Before returning the thing, Decent Exposure will run it through the decoration process. Initially, this does nothing, but you can obviously change that:

expose :things, ->{ Thing.all.map{ |thing| ThingDecorator.new(thing) } }
expose :thing, decorate: ->(thing){ ThingDecorator.new(thing) }


You can pre-save some configuration with exposure_config method to reuse it later.

exposure_config :cool_find, find: ->{ very_cool_find_code }
exposure_config :cool_build, build: ->{ very_cool_build_code }

expose :thing, with: [:cool_find, :cool_build]
expose :another_thing, with: :cool_build

Rails Mailers

Mailers and Controllers use the save decent_exposure dsl.

Example Mailer

class PostMailer < ApplicationMailer
  expose(:posts, -> { Post.last(10) })

  def top_posts
    @greeting = "Top Posts"
    mail to: "to@example.org"

  def featured_post(id:)
    @greeting = "Featured Post"
    mail to: "to@example.org"

Rails Scaffold Templates

If you want to generate rails scaffold templates prepared for decent_exposure run:

rails generate decent_exposure:scaffold_templates [--template_engine erb|haml]

This will create the templates in your lib/templates folder.

Make sure you have configured your templates engine for generators in config/application.rb:

# config/application.rb
config.generators do |g|
  g.template_engine :erb

Now you can run scaffold like:

rails generate scaffold post title description:text


  1. Fork it (https://github.com/hashrocket/decent_exposure/fork)
  2. Create your feature branch (git checkout -b my-new-feature)
  3. Commit your changes (git commit -am 'Add some feature')
  4. Push to the branch (git push origin my-new-feature)
  5. Create a new Pull Request


Hashrocket logo

Decent Exposure is supported by the team at Hashrocket, a multidisciplinary design & development consultancy. If you'd like to work with us or join our team, don't hesitate to get in touch.