end end endWe can ignore the outside_app part as it is only used when rendering a file.
So for each prefix we go across paths and we try to find all matching templates on that path (from which we will take the first result afterwards in the find method).
But what are the values for prefixes and paths ?prefixes will look like this ["users", "application", "base"] because that is how it will get initialized by default (with the controller_path) in the ViewPath module:# Override this method in your controller if you want to change paths prefixes for finding views.
# Prefixes defined here will still be added to parents' <tt>.
def local_prefixes [controller_path]endpaths on the other hand will look like [#Resolver:0x0000.
] starting by a Resolver matching to our app, but with potentially a lot of entries, depending on our number of gems.
What is happening is:in a first loop it will not match our partial path because users does not mach anything in the path of our partial,on the second loop it will “magically” match (because the “application” coming from the hierarchy of our controller will match the “application” of the path we chose for the template).
And the problem is that when we have a big rails application (views, partials, gems that add to the view_paths, etc.
), going through that prefix loop once can be “very costly”.
The Solution(s)= render "application/top_bar"While not prefixing our partial can be very practical and limit oversized line length and repetition, getting used to the “Rails magic” underneath that might open us to that “semantic error” and performance problem.
So the possible solution are:always prefixing partials,banish “base” and “application” from the view path (use some other folder name),be careful to always prefix our partial name when they are in these special paths (“application” and “base”).
In a “real life application”So of course, all this does not come out of nowhere, but from a performance optimization run on a big Rails 5 application.
This Rails app has more than 3000 files in app/views (about 1/3 of these being partials), and 18 “path resolvers” at run-time.
On developmentWe could observe a drop from 100ms to 6ms on each concerned render call.
We could also notice that there was no caching involved in the partial rendering (the main reason being that there is no need for any in most cases)On productionBecause we had this problem mainly on our template layouts, the overall impact was quite big, and most request did benefit from the fix, as for example our UsersController.
the application template breakdownoverall performance impact on that controller requestsAs these graphs show, with an average request taking previously around500ms, nearly a third of that was taken to find partials.
Finding a partial (going through each path of an application and for each running the resolver) is a CPU intensive process (there is I/O involved, but it seems to be cached at some point inside the path resolver).
So we could even see a small dent in the average CPU load of our servers.
.. More details