Pinning plugin and theme versions for older Discourse installs (.discourse-compatibility)

:open_book: Background

Sometimes themes/plugins need to make changes which are only compatible with the latest version of Discourse. In that situation, older versions of Discourse can be instructed to use an older ‘pinned’ version of the plugin.

This is achieved using a .discourse-compatibility file in the root of a theme/plugin repository. It’s a YAML file where keys specify the Discourse core version, and the values represent the associated version of your theme/plugin.

Discourse core versions can be specified using the <= and < operators. <= is the default for historical reasons, but generally it makes more sense to use <.

:pushpin: Pinning a theme/plugin version

For example, if Discourse core makes a change during 3.2.0.beta2-dev and your plugin/theme starts depending on it, then you would add an entry to the .discourse-compatibility file like this:

< 3.2.0.beta2-dev: abcde

where abcde is a reference to the ‘legacy’ commit hash of your plugin which should be used on older versions of Discourse.

Now anyone using an older version of Discourse (e.g. 3.2.0.beta1, or 3.1.4) will use version abcde of your theme/plugin. Anyone on 3.2.0.beta2-dev or above will continue using the latest version.

:clipboard: Multiple Entries

Over time, you can add multiple lines to the .discourse-compatibility file. Discourse will always choose the ‘lowest’ specification which matches the current Discourse core version. The order of the lines in the file doesn’t technically matter, but we recommend putting the newest entries at the top.

:git_merged: ‘Backporting’ changes for old Discourse versions

Let’s imagine a .discourse-compatibility file like this, with two different version specifications pinned to specific plugin commits:

< 3.2.0.beta1-dev: commithashfordiscourse31
< 3.1.0.beta1-dev: commithashfordiscourse30

If you need to ‘backport’ a change to Discourse 3.1, you’d do something like:

  1. Create a branch from the old commit (git checkout -b my_branch_name commithashfordiscourse31)

  2. Commit your change and push to the origin. If you use GitHub’s branch protection features, you may want to protect this branch from accidental deletion

  3. Update the .discourse-compatibility file on the main branch so that it now points to your new commit on the 3.1 support branch

:earth_africa: Real-World Example

Here’s a real .discourse-compatibility file from the discourse-solved plugin. Note that, at the time of writing, this still uses the ‘legacy’ syntax without any explicit < or <= operators. Therefore, each line is automatically interpreted as <=.