Data fetching using LiveComponents

By Benjamin Milde

A common occurance in my phoenix applications is the need to render a list of some items – say companies in a job board application. Let's start out simple by using the phoenix generators to build up the first bunch of boilerplate for those companies:

mix phx.gen.live JobBoard Company companies name:string website:string

This does generate a liveview module MyAppWeb.CompanyLive.Index, which for the liveaction :index can render a nice list of companies.

But usually things do not stop here. Let's say the next requirement is adding the number of jobs published per company in that table on the index page. A quick look into the liveview module will reveal that it calls into its related context module to fetch companies' data for rendering.

def list_companies() do
Repo.all(Company)
end

Preloading

Assuming jobs are related to companies in the database using foreign keys one obvious way of handling the requirement would be to preload jobs from the database and count them up in the template:

def list_companies() do Company 

|> Repo.all()


|> Repo.preload(:jobs)
end

<td><%= Enum.count(company.jobs) %></td>

This however has the downside of loading much more data from the database than required. So another approach could be loading the number of jobs directly from the database instead of counting jobs within elixir.

Subquery

def list_companies() do
jobs_by_company =
from j in Jobs,
group_by: j.company_id,
select: %{
company_id: j.company_id,
job_count: count(j.id)
}

companies_query =


from c in Company,
join: jc in subquery(jobs_by_company),
on: c.id == jc.company_id,
select: %{c | job_count: jc.job_count}

Repo.all(companies_query)


end

field :job_count, :integer, virtual: true

<td><%= company.job_count %></td>

This works great. The database sums up the number of jobs per company and it's simply stored in a virtual field on the company schema. No longer are whole jobs loaded just to count them up.

Multiple collections

Let's imagine another requirement. The JobBoard in the meantime got a new feature: people can submit rating for companies, any company not just the ones publishing jobs. Still the companies index shall show the average rating of each company. That rating itself is part of a completely different context and there's no foreign key relationship on the database level. Querying that separate context from within the JobBoard context doesn't feel like a great solution. Those companies are only related to ratings on the UI level.

Given the last approach is no longer a great one the data loading needs to become multiple steps:

companies = JobBoard.list_companies()
ratings = Comparator.list_ratings(companies)

This seems simple, but is a bit convoluted on the template level:

<%= for company <- @companies do %>
<% rating = Map.get(@ratings, company.id) %>

<% end %>

One would love to just be able to do company.rating instead, but there is no :rating field on the company schema. Searching the ratings for the current company in the iteration each time doesn't seem nice.

LiveComponent

The approach I recently adopted is using live components to compose the various places to fetch data from. Instead of taking all companies, loading all the related ratings in one place, just to bring together individual companies with their individual rating in a completely other place – live components allow for all that to happen in one place.

<%= for company <- @companies do %>
<% live_component @socket, Row, id: company.id, company: company %>
<% end %>
defmodule MyAppWeb.CompanyLive.IndexRowComponent do
use MyAppWeb, :live_component

@impl true


def render(assigns) do
~L
""" <tr> <td><%= @company.name %></td> <td><%= @rating %></td> </tr>

"""


end

@impl true


def preload(list_of_assigns) do
companies = Enum.map(list_of_assigns, & &1.company)
ratings = Comparator.list_ratings(companies)

Enum.map(list_of_assigns, fn assigns ->


Map.put(assigns, :rating, Map.get(ratings, assigns.company))
end)
end
end

The preload/1 callback for those components allows for data fetching to happen on the whole set of companies – preventing N+1 query issues – while the template simply works on per company data, which does hold a simple @rating. No need to deal with mapping individual ratings back to companies in the template itself.

Besides making templates cleaner I also like the idea of letting the liveview itself only deal with fetching companies, which is it's job. While metadata is only added if actually needed. Imagine the rating column is not always enabled. If the component loading ratings is not rendered nothing is loaded. It allows in my opinion for a much better locality as there's not just one "controller" (the liveview) and one view layer (the template), but it's layers of components, which load just the data they themselves need.

Having the preload/1 callback the danger of n+1 queries within loops is mitigated. The only downside left however is multiple different components fetching the same data on demand. Here n would be the number of components though instead of the number of loop iterations.