PluckMap::Presenter
This library provides a DSL for presenting ActiveRecord::Relations without instantiating ActiveRecord models. It is useful when a Rails controller action does little more than fetch several records from the database and present them in some other data format (like JSON or CSV).
Table of Contents
- Why PluckMap?
- Usage
- Defining attributes to present
- Relationships
- Presenting Records
- Installation
- Requirements
- Development & Contributing
Why PluckMap?
Suppose you have an action like this:
def index
messages = Message.created_by(current_user).after(3.weeks.ago)
render json: messages.map { |message|
{ id: message.id,
postedAt: message.created_at,
text: message.text } }
end
☝️ This instantiates a Message
for every result, gets the attributes out of it, and then immediately discards it.
We can skip that unnecessary instantiation by using pluck
:
def index
messages = Message.created_by(current_user).after(3.weeks.ago)
render json: messages.pluck(:id, :created_at, :text)
.map { |id, created, text|
{ id: id,
postedAt: created_at,
text: text } }
end
In a simple benchmark, the second example is 3× faster than the first and allocates half as much memory. 🚀 (Mileage may vary, of course, but in real applications with more complex models, I've gotten more like a 10× improvement at bottlenecks.)
One drawback to this technique is its verbosity — we repeat the attribute names at least three times and changes to blocks like this make for noisy diffs:
def index
messages = Message.created_by(current_user).after(3.weeks.ago)
- render json: messages.pluck(:id, :created_at, :text)
+ render json: messages.pluck(:id, :created_at, :text, :channel)
- .map { |id, created, text|
+ .map { |id, created, text, channel|
{ id: id,
postedAt: created_at,
- text: text } }
+ text: text,
+ channel: channel } }
end
PluckMap::Presenter
gives us a shorthand for generating the above pluck-map pattern. Using it, we could write our example like this:
def index
messages = Message.created_by(current_user).after(3.weeks.ago)
presenter = PluckMap[Message].define do |q|
q.id
q.postedAt select: :created_at
q.text
q.channel
end
render json: presenter.to_h(messages)
end
Using that definition, PluckMap::Presenter
dynamically generates a .to_h
method that is implemented exactly like the example above that uses .pluck
and .map
.
This DSL also makes it easy to make fields optional:
def index
messages = Message.created_by(current_user).after(3.weeks.ago)
presenter = PluckMap[Message].define do |q|
q.id
q.postedAt select: :created_at
q.text
- q.channel
+ q.channel if params[:fields] =~ /channel/
end
render json: presenter.to_h(messages)
end
How is this different from Jbuilder?
Jbuilder gives you a similar DSL for defining JSON to be presented but it operators on instances of ActiveRecord objects rather than producing a query to pluck just the values we need from the database.
Usage
Defining Attributes to Present
Syntax
Define attributes using either of these syntaxes:
-
Without the block variable
presenter = PluckMap[Book].define do title end
-
With the block variable
presenter = PluckMap[Book].define do |q| q.title end
Apart from the repetition of the block variable, the difference between the two styles is the value of self
within the block. In the first case, self
will be PluckMap::AttributesBuilder
. In the second, self
will be the containing object. The former is less repetitious but the latter can be useful if you want to refer to local methods or instance variables in the context.
:as
and :select
👇 This will construct a query to select books.title
from the database and present the value of each title with the key (or column name) "title"
:
presenter = PluckMap[Book].define do
title
end
There are two ways to change the name of the key that is presented. Both of the following examples will select authors.first_name
from the database and present it as "firstName"
:
-
Using
:as
presenter = PluckMap[Author].define do first_name as: "firstName" end
-
Using
:select
presenter = PluckMap[Author].define do firstName select: :first_name end
You can also pass raw SQL expressions to :select
:
presenter = PluckMap[Person].define do
name select: Arel.sql("CONCAT(first_name, ' ', last_name)")
end
:map
In the example above, we constructed name
from first_name
and last_name
with a SQL expression. There are many reasons why we might want to process values before presenting them. When possible, it's usually more efficient to do this work in the query itself, but there are times when it's necessary or expedient to do it in Ruby. Use :map
to process values returned from the query before they are presented.
Here are a couple of examples:
-
Constructing
"name"
with:map
:presenter = PluckMap[Person].define do name select: %i[ first_name last_name ], map: ->(first, last) { "#{first} #{last}" } end
-
Formatting phone numbers with
:map
:presenter = PluckMap[Person].define do phoneNumber select: %i[ phone_number ], map: ->(number) { PhoneNumberFormatter.format(number) } end
:value
You can also hard-code a value to be used and it won't be queried from the database. There are two ways of expressing this:
presenter = PluckMap[Person].define do
id
type "Person"
end
presenter = PluckMap[Person].define do
id
type value: "Person"
end
Structured attributes
You can also nest attributes by passing a block to the attribute method:
presenter = PluckMap[Person].define do
parent do
id select: :parent_id
type "Parent"
end
end
Relationships
PluckMap can also describe nested data. There are two special methods in the define
block that introduce child resources:
-
has_one
will treat the resource as a nested object or null -
has_many
will treat the resource as an array of nested objects (which may be empty)
The first argument to either of these methods is the name of an association on the presented model.
You can use either of these methods with any kind of ActiveRecord relation (belongs_to
, has_one
, has_many
, has_and_belongs_to_many
), although it generally makes more sense to use has_one
with Rails' singular associations and has_many
with Rails' plural associations.
has_one
In the example below, assume
class Book < ActiveRecord::Base
belongs_to :author
end
This presenter 👇 selects the title of every book as well as its author's name:
presenter = PluckMap[Book].define do
title
has_one :author do
name
end
end
(We can also write it using block variables, if that's easier to read.)
presenter = PluckMap[Book].define do |book|
book.title
book.has_one :author do |author|
author.name
end
end
Attributes defined for a relationship support all the same features as attributes defined at the root level.
has_many
We can present the reverse of the above example with has_many
. This example will select a list of authors and, for each, a list of the books they wrote:
presenter = PluckMap[Author].define do
name
has_many :books do
title
end
end
scopes
An optional second argument to both has_one
and has_many
is a scope block that you can use to modify the query that would select the associated records. You can use any of ActiveRecord's standard querying methods inside the scope block.
In this example, we've altered our last presenter to ensure that books are listed alphabetically:
presenter = PluckMap[Author].define do
name
has_many :books, -> { order(title: :asc) } do
title
end
end
Presenting Records
:to_h
Once you've defined a presenter, pass an ActiveRecord::Relation
to to_h
to get an array of hashes:
presenter = PluckMap[Person].define do
id
type value: "Person"
end
presenter.to_h(Person.where(id: 1)) # => [{ id: 1, type: "Person" }]
:to_json
and :to_csv
You can .map
that array to construct whatever document you need, but PluckMap implements two methods that are optimized for generating JSON and CSV:
presenter.to_json(Person.where(id: 1)) # => '[{"id":1,"type":"Person"}]'
presenter.to_csv(Person.where(id: 1)) # => "id,type\n1,Person"
Custom Presenters
You can define new (or override existing) presenter methods by mixing modules into PluckMap::Presenter
. Here's an example of how you might create a presenter that produces an Excel document using an imaginary Excel::Document
library:
module PluckToXlsxPresenter
def to_excel(query)
# Every presenter method accepts an ActiveRecord::Relation
# and passes it to `pluck` which yields the results.
pluck(query) do |results|
# Use an imaginary Excel gem that has an Excel::Document object
spreadsheet = Excel::Document.new
# Fill in a Header row
# `attributes` is a method on `PluckMap::Presenter` that describes
# the attributes you defined when you constructed the presenter.
attributes.each_with_index do |attribute, i|
spreadsheet.cell[0, i] = attribute.name
end
# Results is an array of rows (Rows are an array of values)
results.each_with_index do |values, row_number|
attributes.each_with_index do |attribute, column_number|
# `attribute.exec` will pick the right values from the row
# and perform any required processing.
spreadsheet.cell[row_number + 1, column_number] = attribute.exec(values)
end
end
spreadsheet.render # `pluck` returns the result of the block
end
end
end
PluckMap::Presenter.send :include, PluckToXlsxPresenter
Installation
Add this line to your application's Gemfile:
gem "pluck_map"
And then execute:
$ bundle
Or install it yourself as:
$ gem install pluck_map
Requirements
The gem's only runtime requirement is:
- activerecord 4.2+
It supports these databases out of the box:
- PostgreSQL 9.4+
- MySQL 5.7.22+
- SQLite 3.10.0+
(Note: the versions given above are when certain JSON aggregate functions were introduced in each supported database. PluckMap
's core behavior will work with earlier versions of the database above but certain features like optimizations to to_json
and relationships require the specified versions.)
Development
After checking out the repo, run bin/setup
to install dependencies. Then, run bundle exec rake
to run the tests. You can also run bin/console
for an interactive prompt that will allow you to experiment.
To install this gem onto your local machine, run bundle exec rake install
. To release a new version, update the version number in version.rb
, and then run bundle exec rake release
, which will create a git tag for the version, push git commits and tags, and push the .gem
file to rubygems.org.
Contributing
Bug reports and pull requests are welcome on GitHub at https://github.com/boblail/pluck_map.