Recently I bumped into Rails Survey 2020 results and saw the top 10 gems frustrate one the most. On the 5th place, there was the Active Admin gem. I would not say this was an unexpected result. I often come across the opinion that Active Admin is only suitable for a 15-minute blog, but there is much more with this library.

Here are some approaches my colleagues and I take when working with Active Admin.

An Unofficial Active Admin Guide

Active Admin is based on several libraries, among which I would highlight arbre, formtastic, inherited_resources, and ransack. Each of them is responsible for its part and deserves separate consideration. Let’s start alphabetically with the library extracted from Active Admin itself.

Arbre: custom components

One of the problems with Active Admin is rapidly growing resource files: filters, additional actions, templates, forms, and so on – everything is in one file. I can hear a lonely moan somewhere in the distance: “what about the single responsibility principle?” There is none. Let me show you how you can isolate some templates in separate classes.

Arbre is a library for defining templates using Ruby objects. Here’s an example of a basic page written with Arbre DSL:

html do head do title('Welcome page') end body do para('Hello, world') end

DSL can be extended with components. For example, in Active Admin, these are tabs, table_for, paginated_collection, and even resource pages themselves. Next, we’ll dive in and explore the structure of the basic Arbre component.

Arbre: hello world component

Like all Arbre components, our Admin::Components::HelloWorld inherits from Arbre::Component class:

# app/admin/components/hello_world.rb
module Admin module Components class HelloWorld < Arbre::Component builder_method :hello_world def build(attributes = {}) super(attributes) text_node('Hello world!') add_class('hello-world') end def tag_name 'h1' end end end

Starting from the top: builder_method defines a method to create a component using DSL. Arguments passed to the component will be passed to the #build method.

Each Arbre component is a separate DOM element (similar to the way modern frontend frameworks work, only dates back to 2012). All components are rendered as div DOM elements by default. You can override #tag_name method to change this behavior. As you might guess, #add_class method adds a class attribute to the root DOM element.

At this point, the only thing left is to call our new component. For example, let’s do this in app/admin/dashboard.rb:

# app/admin/dashboard.rb
ActiveAdmin.register_page 'Dashboard' do menu priority: 1, label: proc { I18n.t('active_admin.dashboard') } content do hello_world end

Hello world component

Up next is an example of a small refactoring of the admin panel using a custom component.

Arbre: (almost) a real-life example

To understand how to use Arbre in a production environment, let’s assume that we have a blog with posts (Post) and comments (Comment) with a 1:M relationship. We need to display the last ten comments on the show page of a post.

# app/admin/posts.rb
ActiveAdmin.register Post do permit_params :title, :body show do attributes_table(:body, :created_at) panel I18n.t('active_admin.posts.new_comments') do table_for resource.comments.order(created_at: :desc).first(10) do column(:author) column(:text) column(:created_at) end end end

New comments component

Now we’ll move the table with comments into a separate component. Create a new class and inherit it from ActiveAdmin::Views::Panel. If you create a new component from scratch (as in hello_world example above) and call panel from it, panel will be wrapped by another div, and this will probably break the layout.

Put our new class in app/admin/components/posts/new_comments.rb, since Active Admin automatically requires everything inside app/admin/**/*:

# app/admin/components/posts/new_comments.rb
module Admin module Components module Posts class NewComments < ActiveAdmin::Views::Panel builder_method :posts_new_comments def build(post) super(I18n.t('active_admin.posts.new_comments')) table_for last_comments(post) do column(:author) column(:text) column(:created_at) end end private def last_comments(post) post.comments .order(created_at::desc) .first(10) end end end end

Replace panel in app/admin/posts.rb with our new component and pass resource object as an argument:

# app/admin/posts.rb
ActiveAdmin.register Post do permit_params :title, :body show do attributes_table(:body, :created_at) posts_new_comments(resource) end

Awesome! Note that resource is also available from the component’s context. However, by explicitly passing resource to the builder, we achieve loose coupling, which allows us to reuse the component in the future.

Speaking of reuse, we can extract everything from the show block (as well as other template blocks) into partial:

# app/admin/posts.rb
ActiveAdmin.register Post do show do render('show', post: resource) end
# app/views/admin/posts/_show.html.arb
panel(ActiveAdmin::Localizers.resource(active_admin_config).t(:details)) do attributes_table_for(post, :body, :created_at)
end posts_new_comments(post)

Note: you can use familiar .erb and other templating engines instead of .arb.

Arbre: what’s next

First of all, I do advise you to read Active Admin components’ official documentation.

Besides, you can read code for base components from arbre and activeadmin components. Those are the components your custom ones will inherit from. Also, check out the gem activeadmin_addons – it has a lot of interesting custom components.

Well, if you still write code with errors (this is still a thing for some reason), check out how to test custom components.

Formtastic: custom forms

Formtastic is a library for describing forms using DSL. The simplest form looks like this:

semantic_form_for(object) do |f| f.inputs f.actions

In the example, Formtastic automatically extracts all attributes from the passed object and inserts them into a form with default input types. A list of available input types can be found in the README. Like Arbre, Formtastic can be extended by creating custom component classes. To understand the basics, we’ll create a hello world component.

Formtastic: hello world component

By analogy with Arbre components, we’ll place the new class in app/admin/inputs:

# app/admin/inputs/hello_world_input.rb
class HelloWorldInput include Formtastic::Inputs::Base def to_html "Input for ##{object.public_send(method)}" end

To apply a new input type, simply specify its name as :as parameter, for example:

# app/admin/posts.rb
ActiveAdmin.register Post do form do |f| f.inputs do f.input(:id, as: :hello_world) f.input(:title) f.input(:body) end f.actions end

Hello world input

All the parameters required to draw the form (including object and method) are passed to #initialize defined in the module Formtastic::Inputs::Base. The #to_html method is responsible for rendering the input.

The example may seem useless, but in fact, we use it to render read-only fields. To turn our hello world to a useful read-only input, we only need to add a couple of methods from Formtastic::Inputs::Base:

# app/admin/inputs/hello_world_input.rb
class HelloWorldInput include Formtastic::Inputs::Base def to_html input_wrapping do label_html << template.format_attribute(object, method) end end

Read-only input

input_wrapping from Formtastic::Inputs::Base::Wrapping module is responsible for input wrapping and rendering error output and hints. label_html from Formtastic::Inputs::Base::Labelling module renders the label for input. These two helpers instantly turn our hello world into a combat-applicable input. Last but not least, format_attribute from ActiveAdmin::ViewHelpers::DisplayHelper is a helper method for rendering input values.

Now we’ll move to a slightly more complex example that demonstrates how to integrate a JavaScript library with a form.

Formtastic: (almost) a real-life example

We’ll take another made-up example to demonstrate how to work with HTML, CSS, and JS. In other words, it will cover all the steps of writing a new input.

Say we have received a request from our blog editor: when writing a post, he would like to see the number of words directly in the input form. As you know, the world of JavaScript has libraries for everything, and there is one for our task too: Countable.js. Let’s take the standard input for text (textarea) and extend it with a word counter.

To implement the new input, we need:

  • take the existing text input and add div to it to output the number of words;
  • add CSS styles for the new div;
  • initialize Countable.js and use it to write the number of words in the new div.

Firstly, we need to create a new class and inherit it from Formtastic::Inputs::TextInput. Add attribute class="countable-input" to the element textarea and define a new empty div with the attribute class="countable-content" next to it:

# app/admin/inputs/countable_input.rb
class CountableInput < Formtastic::Inputs::TextInput def to_html input_wrapping do label_html << builder.text_area(method, input_html_options.merge(class: 'countable-input')) << template.content_tag(:div, '', class: 'countable-content') end end

Now, have a look at what we have added. input_html_options is a parent class method, which returns HTML attributes for the input. builder - is an instance of the class ActiveAdmin::FormBuilder, inherited from ActionView::Helpers::FormBuilder. template is the context in which the templates are executed (basically, a huge set of view-helpers). Thus, if we need to create a piece of form, we’ll call builder. While if we want to use something like link_to, template will help us.

Let’s call the Countable.js library: put it into vendor/assets/javascripts directory and add a simple .js file which will call Countable.js and throw the information into div.countable-content (please don’t be harsh on this piece of spaghetti code):

// app/assets/javascripts/inputs/countable_input.js
//= require countable.min.js const countable_initializer = function () { $('.countable-input').each(function (i, e) { Countable.on(e, function (counter) { console.log( $(e).closest('.countable-content') ) $(e).parent().find('.countable-content').html('words: ' + counter['words']); }); });
} $(countable_initializer);
$(document).on('turbolinks:load', countable_initializer);

Now we include it in app/assets/javascripts/active_admin.js:

// app/assets/javascripts/active_admin.js
// ... //= require inputs/countable_input

The last step is to add a CSS file and include it in app/assets/stylesheets/active_admin.scss:

// app/assets/stylesheets/inputs/countable_input.scss
.countable-content { float: right; font-weight: bold;
// app/assets/stylesheets/active_admin.scss
@import "inputs/countable_input";

That’s it! Our new input is ready. Now we can call it in the form:

# app/admin/posts.rb
ActiveAdmin.register Post do form do |f| f.inputs do f.input(:id, as: :hello_world) f.input(:title) f.input(:body, as: :countable) end f.actions end

Custom input

This is how one can make custom components for forms, like file loaders or inputs with tricky autofill. There is a bit more code in such components, but the approach remains the same.

Formtastic: Warmest greetings to the DRY principle

As with the Arbre components, forms can be put into partial’s, although the syntax is slightly different:

# app/admin/posts.rb
ActiveAdmin.register Post do form(partial: 'form')
# app/views/admin/posts/_form.html.arb
active_admin_form_for(resource) do inputs(:title, :body) actions

The disadvantage of the approach is that forms are placed somewhere deep in the views directory. In my opinion, this makes code navigation a bit more complicated, but that is a matter of taste.

Formtastic: what’s next

Formtastic is a pretty big library, and I would highly recommend reading the detailed README to get acquainted with all customization options. It will also be useful to see the already mentioned activeadmin_addons gem. There are lots of additional inputs in this library that are worth being checked out.

Needless to say, although I have divided Formtastic and Arbre into different blocks of the article, they perfectly work together. You can even create forms or parts of forms as Arbre-components.

Inherited Resources: custom controllers

To understand where does magical resource come from, how to change the saving behavior, and much more, we need to get acquainted with another gem.

Inherited Resources is a library designed to reduce the amount of CRUD boilerplate in controllers.

On the one hand, the library is pretty simple, but on the other hand, it is quite comprehensive. So let’s have a quick look at a few useful methods:

class PostsController < InheritedResources::Base respond_to :html respond_to :json, only: :index actions :index, :new, :create def update resource.updated_by = current_user update! { posts_path } end

.respond_to is responsible for the available formats. All .respond_to calls are stacked rather than overriding each other. To reset the formats we need the .clear_respond_to method.

.actions defines available CRUD methods (index, show, new, edit, create, update, and destroy).

resource is one of the available helpers:

resource #=> @post
collection #=> @posts
resource_class #=> Post

Finally, #update! is just alias for #update which can be used instead of super when overloading methods.

Next, we’ll have a look at the .has_scope method in action. Let’s presume that the post class has a defined scope :published:

class Post < ApplicationRecord scope :published, -> { where(published: true) }

In this case, we can use the .has_scope method in the controller:

class PostsController < InheritedResources::Base has_scope :published, type: :boolean

.has_scope adds filtering using query-parameters. In the given example, we can apply the scope :published by viewing the collection at URL /posts?published=true.

A detailed description of these and other features of the library can be reached at the rich README. I say we stop here and finally move to the interaction with Active Admin.

Inherited Resources: controller modifications

All Active Admin controllers are inherited from InheritedResources::Base, which means that we can modify their behavior using library methods. For example, here is how the list of available controller actions is defined:

# app/admin/posts.rb
ActiveAdmin.register Post do actions :all, :except => [:destroy]

Great, we removed delete action from the article. It seems to be obvious: we use the Active Admin resource as a controller. But let’s not jump to conclusions yet and try to add another feature.

By default, Active Admin includes a rendering of all pages as HTML, JSON, and XML (index is also available in CSV format). Let’s try to get rid of XML rendering for our page using methods that we’ve already learned:

# app/admin/posts.rb
ActiveAdmin.register Post do clear_respond_to respond_to :html, :json respond_to :csv, only: :index


Oh, now we got an error undefined method 'clear_respond_to' for #<ActiveAdmin::ResourceDSL>.

# app/admin/posts.rb
ActiveAdmin.register Post do controller do clear_respond_to respond_to :html, :json respond_to :csv, only: :index end

Voila, now localhost:3000/admin/posts.xml returns an error. And what about modifying the action’s behavior?

Inherited Resources: method overloading

Assume that when saving we need to set the attribute post#created_by_admin. To do this, we’ll take advantage of the #create method overloading feature:

# app/admin/posts.rb
ActiveAdmin.register Post do controller do def create build_resource @post.created_by_admin = true create! end end

We call build_resource, a method that initializes a new object and assigns it to the @post variable. Next, set the attribute created_by_admin and call create! (aka super) which continues to operate on the @post variable we created.

Note: be careful with the helpers. Inherited Resources actively uses instance variables. In the example above, it helped us to create and modify the object, but when used carelessly, the results may be unexpected (I learned that the hard way).

Now let’s take a few steps back to the point where we’ve turned off XML rendering of articles. What if we want to remove XML rendering from all the resources? We wouldn’t write the same code in every new resource, would we?

Inherited Resources: base controller modifications

No, we wouldn’t! Let’s create a module that will adjust the ActiveAdmin::ResourceController class behavior:

# lib/active_admin/remove_xml_rendering_extension.rb
module ActiveAdmin module RemoveXmlRenderingExtension def self.included(base) base.send(:clear_respond_to) base.send(:respond_to, :html, :json) base.send(:respond_to, :csv, only: :index) end end

An extensible class will be passed to the .included method, with all needed modifications applied. We will use the Active Admin initializer and connect the new module to ActiveAdmin::ResourceController:

# config/initializers/active_admin.rb
require 'lib/active_admin/remove_xml_rendering_extension' ActiveAdmin::ResourceController.send( :include, ActiveAdmin::RemoveXmlRenderingExtension
# ...

A bit of metaprogramming magic with #include and #included, and here you go! Now no resource would respond to the .xml format.

By the way, if you think that #prepend, #include, and #extend methods are only useful to pass tricky interview questions, that’s quite wrong. When it is necessary to modify the code of the external library, such approaches are often the only available tool.

Inherited Resources: what’s next

First of all, take a good look at the detailed README. In addition, pay attention to how the controllers are organized in Active Admin, notice the authorization logic, and other little things like additional helpers.

Ransack: custom filters

By default, on each index page, Active Admin provides a powerful block with filtering, from which I often have to remove something rather than add something new. But this filtering block is just the tip of the iceberg called Ransack.

Ransack – a library for creating search forms, which allows you to build complex SQL queries by interpreting the passed parameter names. It sounds complicated, but I’m sure the example will quickly give you an understanding of what I am talking about.

For example, suppose that we need to filter blog posts (post) by a substring in the title (title). With Ransack we can do so like this:

Post.ransack(title_cont: 'sharp knives').result

The postfix _cont is one of the many predicates available in Ransack. Predicates determine which SQL query is to be generated for search. You can read more about all available predicates in the official wiki.

Now let’s make it a bit more complicated: a customer asked us to add a filter that would allow searching by substring of title and/or body (body). With Ransack it is as simple as that:

Post.ransack(title_or_body_cont: 'active admin').result

In addition, Ransack allows you to search for records by referring to associated models. For example, let’s add search by comments (Comment#text):

Post.ransack(comments_text_cont: 'I hate type annotations!').result

As you might guess, such things can grow quickly. Using complex parameters in several places can lead to problems as well. Ransack suggests using #ransack_alias as a solution. Let’s add filtering by an author name to search by comment and give it a short alias: comments which in the future can be used with the predicates we need:

# app/models/post.rb
class Post < ActiveRecord::Base has_many :comments ransack_alias :comments, :comments_text_or_comments_author
end Post.ransack(comments_cont: 'Matz').result

Now that we have learned how Ransack allows us to structure requests. Let’s finally move to how we can use it in Active Admin.

Ransack: combined filters

Let’s take the example above and use them to filter the Active Admin resource:

# app/admin/posts.rb
ActiveAdmin.register Post do preserve_default_filters! filter :title_or_body_cont, as: :string, label: I18n.t('active_admin.filters.title_or_body_cont') filter :comments, as: :string

Combined filter

Basically, that’s it, very straightforward. The only thing I’d like to note is the #preserve_default_filters! method which renders default filters.

Ransack: scope-filters

By default, Ransack allows you to filter by all attributes and relationships in the model. It can be dangerous from a security point of view, so please note that it is possible to restrict access to certain fields and links using the ransackable_attributes, ransackable_associations, and ransackable_scopes methods. I would leave authorization issues outside the scope of the article (especially since Active Admin has a detailed section in its documentation), so let’s only pay attention to the ransackable_scopes method.

Unlike other authorization methods, ransackable_scopes doesn’t allow using any scope by default. Thus, to be able to filter by scope (or any other method of the model class), you need to return its name from .ransackable_scopes.

For example, let’s add a filter by the number of comments using scope:

# app/models/post.rb
class Post < ActiveRecord::Base has_many :comments scope :comments_count_gt, (lambda do |comments_count| joins(:comments) .group('') .having('count( > ?', comments_count) end) def self.ransackable_scopes(auth_object = nil) [:comments_count_gt] end

Note auth_object: in theory, this is the object by which you can define an authorization strategy. I would expect current_user to be passed here, but Active Admin does not do it.

We added a scope and returned its name to .ransackable_scopes, the only thing left is to add a filter to the Active Admin resource:

# app/admin/posts.rb
ActiveAdmin.register Post do filter :comments_count_gt, as: :number, label: I18n.t('active_admin.filters.comments_count_gt')

Comments count filter

There’s one little thing left: if we try to filter all the articles with two or more comments, everything would be fine, but if we try to submit 1, we would get an error:


It is a type conversion that Ransack does for historical reasons. To disable this questionable feature, we should add an initializer with the specified parameter sanitize_custom_scope_booleans:

# /config/initializers/ransack.rb
Ransack.configure do |config| config.sanitize_custom_scope_booleans = false

There you go, now the filter works even if we submit 1 as an argument, and we know how to use scope-based filters.

Ransack: what’s next

First of all, you should take a look at Active Admin’s documentation regarding filters. You can continue your overview with the official README and wiki, where, among other things, you can find view-helpers to create custom search forms.

For especially complicated cases, you can consider learning how to create custom predicates and Ransackers - extensions that convert parameters directly into Arel (internal library ActiveRecord, used to build SQL queries).


I hope that the article allowed you to look at Active Admin from a new perspective, and maybe even inspired you to refactor a class or two in your projects.

I tried not to repeat the official Active Admin documentation, which describes many interesting features of the library, such as authorization and the use of decorators. Therefore make sure to check it once again.

Russian version of the article is published in the Domclick blog.