Convert a partial to method/block for speed

Question

I have a loop that renders a partial

1000.times do |i|
  render partial: 'test', locals: {i: i}
end

this is really slow, up to 0.1 ms for foreach render call, even if the partial only prints out i

my_partial = render_to_method(partial: 'test')
1000.times do |i|
  my_partial(locals: {i: i})
end

Now this should be doing the same thing way faster right? But I don't know how to do this.

update:

I've tried the to do it this way:

Haml::Engine.new(File.read(File.expand_path File.dirname(FILE)+"/../table/#{column.macro}/_#{column.display_type}.haml"))
.‌​render(OpenStruct.new({column_value: column_value, object: object})) 

two major drawbacks:

• The path to the views will not watch for fallbacks like it does when you do it with render (parital: 'partial' will look for the parital in the current view dir, in some gems and also in the view/application.
• The Rails view helpers aren't available any more

update 2:

Many of the answers try to solve the problem by using other techniques. I my real application this techniques can't be applied. https://github.com/antpaw/bhf/blob/master/app/views/bhf/pages/_platform.haml#L54‌​ mainly because this line is a dynamic string that sometimes links to partials that don't even exsist in the gem and are defined in the main_app. I wonder why it's so hard to do something that is so basic: 1. grab the view. 2. render it. (3. render it again.)

update 3:

@Flash Gordon suggested this https://gist.github.com/antpaw/d6670c23d6f08e35812e#file-gistfile1-haml-L54

template = lookup_context.find_template "#{column.macro}/#{column.display_type}", ['bhf/table'], true
template.render(self, {column_value: column_value, object: object})

it almost works just having some troubles with locals. But it already feels like the part where you grab the template and the render part is well separated.

Answer 1

Here is an example from one of my previous answers. It's extracted from PartialRenderer sources.

- local_names = [:i]
- partials = {}
- 1000.times do |i|
  - name = 'name_%s' % (i % 10)
  - partials[name] ||= lookup_context.find_template(name, lookup_context.prefixes, true, local_names)
  = partials[name].render(self, i: i)

I'd recommend you to wrap it with a helper method. Keep in mind that locals' names appear here twice: first in local_names as an array and second in hash's keys passed as the second argument of #render method.

Answer 2

I love this question, but it may be a "wrong tool for the job" thing

Basically:

1. if the partial is simple - consider not using a partial at all, the overhead isn't worth it
2. if the partial is complex - then rendering 1000s of things will take TIME out of which partial overhead is just a fraction of, you'll need a plan B

Plan Bs:

1. render 25-50-100.... thingies at first and fetch additional records from the server as the user scrolls - http://railscasts.com/episodes/114-endless-page
2. send a JSON array from the server and let client side JS iterate through the collection and render HTML - this way the partial template can still be rendered in rails and client side JS can do gsub
3. cache everything you possibly can - this doesn't include forms unfortunately which are typically the slowest rendering components
4. ...

PS have you tried out cells? https://github.com/apotonick/cells

Answer 3

This is an interesting proposition and it seems harder than I thought to find information about it. However from what I can tell from the implementation of the response object I would think it should be possible to append (<<) your output to the stream object of the response object. This will be rendered as a string into the response#body as necessary.

The tricky part is to find a good place to define your my_partial method. While it is not commonplace to do this in a View it should still be possible and I think you should have access to the response object directly. Otherwise you might define it in a helper, though you probably do not want to use this code in a controller.

Sorry if this is rather an idea than an answer, but I lack the time to test it properly.

Answer 4

You could add a helper method and use content_tag. This accomplishes your goal of having a method work as a partial.

def my_partial(i)
  content_tag(:div, i, class: 'iterator')
end

Using helper methods with content_tag probably isn't a good solution if you have a lot of content in your partial, for readability's sake.

An alternative recommendation: Instead of rendering the same partial multiple times within a loop, you could render a single partial that includes a loop. If you needed to reuse the partial for only one object, you could just pass the object as an array.

Here's a more real-world example:

# index.html.erb
<%= render partial: 'dogs', locals: {dogs: @dogs} %>

# show.html.erb
<%= render partial: 'dogs', locals: {dogs: [@dog]} %>

# _dogs.html.erb
<% dogs.each do |dog| %>
  <%= dog.name %>
<% end %>

Answer 5

Use :collection option:

= render partial: 'test', collection: 10.times.to_a, as: :i

Docs: http://guides.rubyonrails.org/layouts_and_rendering.html#using-partials

Source: https://github.com/rails/rails/blob/master/actionview/lib/action_view/renderer/partial_renderer.rb#L284

Answer 6

If you need render many times and it is slow, I suggest you do not call render many times. I know, I am captain obvious.

If locals contains simple strings or numbers, try to render your view once, and then just replace parts with actual variables.

text = render(partial: 'test', locals: {i: '$'})

1000.times do |i|
  output[i] = text.gsub('$', i)
end