Sprockets 4.0 was released on October 8th 2019, after several years of beta, congratulations and hooray.

There are a couple confusing things that may give you trouble trying to upgrade to sprockets 4 that aren’t covered very well in the CHANGELOG or upgrade notes, although now that I’ve taken some time to understand it, I may try to PR to the upgrade notes. The short version:

If your Gemfile has `gem ‘sass-rails’, ‘~> 5.0’` in it (or just '~> 5' ), that will prevent you from upgrading to sprockets 4. Change it to `gem ‘sass-rails’, ‘~> 6.0’` to get sass-rails that will allow sprockets 4 (and, bonus, will use the newer sassc gem instead of the deprecated end-of-lifed pure-ruby sass gem). Sprockets 4 changes the way it decides what files to compile as top-level aggregated compiled assets. And Rails (in 5.2 and 6) is generating a sprockets 4 ‘config’ file that configures something that is probably inadvisable and likely to do the wrong thing with your existing app. If you are seeing an error like Undefined variable: $something this is probably affecting you, but it may be doing something non-optimal even without an error. (relevant GH Issue) You probably want to go look at your ./app/assets/config/manifest.js file and turn //= link_directory ../stylesheets .css to //= link 'application.css' . If you are not yet in Rails 6, you probably have a //= link_directory ../javascripts .js , change this to link application.js This still might not get you all the way to compatiblity with your existing setup, especially if you had additional top-level target files. See details below.



The Gory Details

I spent some hours trying to make sure I understood everything that was going on. I explored both a newly generated Rails 5.2.3 and 6.0.0 app; I didn’t look at anything earlier. I’m known for writing long blog posts, cause I want to explain it all! This is one of them.

Default generated Rails 5.2 or 6.0 Gemfile will not allow sprockets 4

Rails 5.2.3 will generate a Gemfile that includes the line:

gem 'sass-rails', '~> 5.0'

sass-rails 5.x expresses a dependency on sprockets < 4 , so won’t allow sprockets 4.0.0.

This means that a newly generated default rails app will never use sprockets 4.0. And you can’t get to sprockets 4.0 by running any invocation of bundle update , because your Gemfile links to a dependency requirement tree that won’t allow it.

The other problem with sass-rails 5.x is it depends on the deprecated and end-of-lifed pure-ruby sass gem. So if you’re still using it (say with a default generated Rails app), you may be seeing deprecation “please don’t use this messages” too.

So some people may have already updated their Gemfile. There are a couple ways you can do that:

You can change the dependency to gem 'sass-rails', '~> 6.0' (or '>= 5.0' ), which is what an upcoming Rails release will probably do)

(or ), which is what an upcoming Rails release will probably do) But sass-rails 6.0 is actually a tiny little wrapper over a different gem, sassc-rails. (which itself depends on non-deprecated sassc instead of deprecated pure-ruby sass). So you can also just change your dependency to gem 'sassc-rails', '~> 2.0' ,

, which you may have already done when you wanted to get rid of ruby-sass deprecation warnings, but before sass-rails 6 was released. (Not sure why they decided to release sass-rails 6 as a very thin wrapper on a sassc-rails 2.x), and have Rails attempt to still generate a Gemfile with sass-rails

Either way, you will then have a dependency requirement tree which allows any sprockets `> 3.0` (which is still an odd dependency spec; 3.0.0 isn’t allowed, but 3.0.1 and higher are? It probably meant `>= 3.0`? Which is still kind of dangerous for allowing future sprockets 5 6 or 7 too…) — anyway, so allows sprockets 3 or 4.

Once you’ve done that, if you do a bundle update now that sprockets 4 is out, you may find yourself using it even if you didn’t realize you were about to do a major version upgrade. Same if you do bundle update somegem , if somegem or something in it’s dependency tree depends on sprockets-rails or sprockets, you may find it upgraded sprockets when you weren’t quite ready to.

Now, it turns out Rails 6.0.0 apps are in exactly the same spot, all the above applies to them too. Rails intended to have 6.0 generate a Gemfile which would end up allowing sass-rails 5.x or 6.x, and thus sprockets 3 or 4.

It did this by generating a Gemfile with a dependency that looks like ~> 5 , which they thought meant `>= 5` (I would have thought so too), but it turns out it doesn’t, it seems to mean the same thing as ~> 5.0 , so basically Rails 6 is still in the same boat. That was fixed in a future commit, but not in time for Rails 6.0.0 release — Rails 6.1 will clearly generate a Gemfile that allows sass-rails 5/6+ and sprockets 3/4+, not sure about a future 6.0.x.

So, Rails 5.2 won’t allow you to upgrade to sprockets 4 without a manual change, and it turns out accidentally Rails 6 won’t either. That might be confusing if you are trying to update to sprockets 4, but it actually (accidentally) saves you from the confusion that comes from accidentally upgrading to sprockets 4 and finding a problem with how top-level targets are determined. (Although if even before sprockets4 came out you were allowing sass-rails 6.x to avoid deprecated ruby-sass… you will be able to get sprockets 4 with bundle update , accidentally or on purpose).

Rails-Sprockets built-in logic for determining top-level compile targets CHANGES depending on Sprockets 3 or 4

The sprockets-rails gem actually has a conditional for applying different logic depending on whether you are using Sprockets 3 or 4. Rails 5.2 or 6 won’t matter; but in either Rails 5.2 or 6, changing from Sprockets 3 to 4 will change the default logic for determining top-level compile targets (the files that can actually be delivered to the browser, and will be generated in your public/assets directory as a result of rake assets:precompile ).

This code has been in sprockets-rails since sprockets-rails 3.0, released in December 2015(!). The preparations for sprockets 4 are a long time coming.

This means that switching from Sprockets 3 to 4 can mean that some files you wanted to be delivered as top-level targets no longer are; and other files that you did not intend to be are; in some cases, when sprockets tries to compile as a top-level target when not intended as such, the file actually can’t be compiled as such without an error, and that’s when you get an error like Undefined variable: $something — it was meant as a sass “partial” to be compiled in a context where that variable was defined, but sprockets is trying to compile it as a top-level target.

rails-sprockets logic for Sprockets 3

If you are using sprockets 3, the sprockets-rails logic supplies a regexp basically saying the files `application.css` and `application.js` should be compiled as top-level targets. (That might apply to such files found in an engine gem dependency too? Not sure).

And it supplies a proc object that says any file that is in your local ./app/assets (or a subdir), and has a file extension, but that file extension is not `.js` or `.css` => should be compiled as a top-level asset.

Actually not just .js and .css are excluded, but anything sprockets recognizes as compiling to .js or .css, so .scss is excluded too.

That is maybe meant to get everything in ./app/assets/images , but in fact it can get a lot of other things, if you happened to have put them there. Say ./app/assets/html/something.html or ./app/assets/stylesheets/images/something.png .

rails-sprockets logic for Sprockets 4

If you are using sprockets-4, sprockets won’t supply that proc or regexp (and in fact proc and regexp args are not supported in sprockets 4, see below), but will tell sprockets to start with one file: manifest.js .

This actually means any file in any subdir of app/assets (maybe files from rails engine gems too?), but the intention is that this refers to app/assets/config/manifest.js .

The idea is that the manifest.js will include the sprockets link , link_directory , and link_tree methods to specify files to treat as top-level targets.

And possibly surprising you, you probably already have that file there, because Rails has been generating that file for new apps for some time. (I am not sure for how long, because I haven’t managed to find what code generates it. Can anyone find it? But I know if you generate a new rails 5.2.3 or rails 6 app, you get this file even though you are using sprockets 3).

If you are using sprockets 3, this file was generated but not used, due to the code in sprockets-rails that does not set it up for use if you are using sprockets 3. (I suppose you could have added it to Rails.application.config.assets.precompile yourself in config/initializers/assets.rb or wherever). But it was there waiting to be used as soon as you switched to sprockets4.

What is in the initial Rails-generated app/assets/config/manifest.js ?

In Rails 5.2.3:

//= link_tree ../images //= link_directory ../javascripts .js //= link_directory ../stylesheets .css

This means:

Anything in your ./app/assets/images , including subdirectories

, including subdirectories Anything directly in your `./app/assets/javascripts` (not including subdirs) that ends in `.js`.

Anything directly in your `./app/assets/stylesheets` (not including subdirs) that ends in `.css` So here’s the weird thing, it actually seems to mean “any file recognized as a CSS” file — file ending in `.scss` get included too. I can’t figure out how this works or is meant to work; Can anyone find better docs for what the second arg to `link_directory` or `link_tree` does or figure it out from the code, and want to share?



Some significant difference between sprockets3 and sprockets4 logic

A initially generated Rails 5.2.3 app has a file at ./app/assets/javascripts/cable.js . It is referenced with a sprockets require from the generated application.js; it is not intended to be a top-level target compiled separately. But a default generated Rails 5.2.3 app, once using sprockets 4 — will compile the cable.js file as a top-level target, putting it in `public/assets` when you do rake assets:precompile . Which you probably don’t want.

It also means it will take any CSS file (including .scss) directly (not in subdir) at ./app/assets/stylesheets and try to compile them as top-level targets. If you put some files here that were only intended to be `imported` by sass elsewhere (say, _mixins.scss ), sprockets may try to compile them on their own, and raise an error. Which can be a bit confusing, but it isn’t really a “load order problem”, but about trying to compile a file as a top-level target that wasn’t intended as such.

Even if it doesn’t raise an error, it’s spending time compiling them, and putting them in your public/assets , when you didn’t need/want them there.

Perhaps it was always considered bad practice to put something at the top-level `./app/assets/stylesheets` (or ./app/assets/javascripts?) that wasn’t intended as a top-level target… but clearly this stuff is confusing enough that I would forgive anyone for not knowing that.

Note that the sprockets-rails code activated for sprockets3 will never choose any file ending in .js or .css as a top-level target, they are excluded. While they are specifically included in the sprockets4 code.

(Rails 6 is identical situation to above, except it doesn’t generate a `link_directory` referencing assets/javascripts, becuase Rails 6 does not expect you will use sprockets for JS, but will use webpacker instead).

I am inclined to say the generated Rails code is a mistake and it probably should be simply

//= link_tree ../images //= link application.js # only in Rails 5.2 //= link application.css

You may want to change it to that. If you have any additional things that should be top-level targets compiled, you will have to configure them seperately…

Options for configuring additonal top-level targets

If you are using Sprockets 3, you are used to configuring additional top-level targets by setting the array at Rails.application.config.assets.precompile . (Rails 6 even still generates a comment suggesting you do this at ./config/initializers/assets.rb ).

The array at config.assets.precompile can include filenames (not including paths), a regexp, or a proc that can look at every potential file (including files in engines I think?) and return true or false.

If you are using sprockets4, you can still include filenames in this array. But you can not include regexps or procs. If you try to include a regexp or proc, you’ll get an error that looks something like this:

`NoMethodError: undefined method `start_with?' for #` ...sprockets-4.0.0/lib/sprockets/uri_utils.rb:78:in `valid_asset_uri?'

While you can still include individual filenames, for anything more complicated you need to use sprockets methods in the `./app/assets/config/manifest.js` (and sprockets really wants you to do this even instead of individual filenames).

The methods available at `link`, `link_directory`, and `link_tree`. The documentation isn’t extensive, but there’s some in the sprockets README , and a bit more in sourcecode in a somewhat unexpected spot.

I find the docs a bit light, but from experimentation it seems to me that the first argument to link_directory and link_tree is a file path relative to the manifest.js itself (does not use “asset load path”), while the first argument to link is a file path relative to some dir in “asset load path”, and will be looked up in all asset load paths (including rails engine gems) and first one found used.

For instance, if you have a file at ./app/assets/images/foo/bar.jpg , you’d want //= load foo/bar.jpg since all subdirs of ./app/assets/ end up in your “asset load path”.

, you’d want since all subdirs of end up in your “asset load path”. I’m not sure where what I’m calling the “asset load path” is configured/set, but if you include a //= load for some non-existent file, you’ll conveniently get the “asset load path” printed out in the error message!

The new techniques are not as flexible/powerful as the old ones that allowed arbitrary proc logic and regexps (and I think the proc logic could be used for assets in dependent engine gems too). So you may have to move some of your intended-as-top-level-targets source files to new locations, so you specify them with the link/link_tree/link_directory functions available; and/or refactor how you are dividing things between what asset files generally.

What went wrong here? What should be fixed?

Due to conditional logic in sprockets 3/4, very different logic for determining top-level targets will be used when you update to sprockets 4. This has affected a lot of people I know, but it may affect very few people generally and not be disruptive? I’m not sure.

But it does seem like kind of a failure in QA/release management, making the upgrade to sprockets 4 not as backwards compat as intended. While this roadblock was reported to sprockets in a 4.0 beta release back in January, and reported to Rails too in May, sadly neither issue received any comments or attention from any maintainers before or after sprockets 4.0 release; the sprockets one is still open, the rails one was closed as “stale” by rails-bot in August.

This all seems unfortunate, but the answer is probably just that sprockets continues to not really have enough maintainers/supporters/contributors working on it, even after schneem’s amazing rescue attempt.

If it had gotten attention (or if it does, as it still could) and resources for a fix… what if anything should be done? I think that Rails ought to be generating the ./app/assets/config/manifest.js with eg //= link application.css instead of //= link_directory ../stylesheets .css .

I think that would be closer to the previous sprockets3 behavior, and would not do the ‘wrong’ thing with the Rails 5.2.3 cable.js file. (In Rails 6 by default sprockets doesn’t handle JS, so cable.js not an issue for sprockets).

file. (In Rails 6 by default sprockets doesn’t handle JS, so not an issue for sprockets). This would be consistent with the examples in the sprockets upgrading guide.

I think/guess it’s basically a mistake, from inconsistent visions for what/how sprockets/rails integration should or would work over many years with various cooks.

Since (by accident) no Rails has yet been released which will use Sprockets 4 (and the generated manifest.js file) without a manual change to the Gemfile, it might be a very good time to fix this before an upcoming Rails release that does. Becuase it will get even more confusing to change at a later date after that point.

The difficulties in making this so now:

I have been unable to find what code is generating this to even make a PR. Anyone?

Finding what code is generating it would also help us find commit messages from when it was added, to figure out what they were intending, why they thought this made sense.

But maybe this is just my opinion that the generated manifest.js should look this way. Am I wrong? Should (and will) a committer actually merge a PR if I made one for this? Or is there some other plan behind it? Is there anyone who understands the big picture? (As schneems himself wrote up in the Saving Sprockets post, losing the context brought by maintainers-as-historians is painful, and we still haven’t really recovered).

Would I even be able to get anyone with commit privs attention to possibly merge a PR, when the issues already filed didn’t get anyone’s attention? Maybe. My experience is when nobody is really sure what the “correct” behavior is, and nobody’s really taking responsibility for the subsystem, it’s very hard to get committers to review/merge your PR, they are (rightly!) kind of scared of it and risking “you broke it you own it” responsibility.

Help us shneems, you’re our only hope?

My other conclusion is that a lot of this complexity came from trying to make sprockets decoupled from Rails, so it can be used with non-Rails projects. The confusion and complexity here is all about the Rails/sprockets integration, with sprockets as a separate and decoupled project that doens’t assume Rails, so needs to be configured by Rails, etc. The benefits of this may have been large, it may have been worth it — but one should never underestimate the complexity and added maintenance burden of trying to make an independent decoupled tool, over something that can assume a lot more about context, and significantly added difficulty to making sprockets predictable, comprehensible, and polished. We’re definitely paying the cost here, I think a new user to Rails is going to be really confused and overwhelemed trying to figure out what’s going on if they run into trouble.