Add Join URI Path Parts as a Ruby TIL

ruby/join-uri-path-parts.md

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+# Join URI Path Parts
+
+The
+[`URI.join`](https://ruby-doc.org/stdlib-2.5.1/libdoc/uri/rdoc/URI.html#method-c-join)
+method seems like a handy way to combine a base URL with some subpath. However,
+there are some subtle gotchas depending on where forward slashes appear in the
+two arguments.
+
+Let's first look at the, in my opinion, desired behavior:
+
+```ruby
+> URI.join("https://example.com/api/v1/", "users")
+=> #<URI::HTTPS https://example.com/api/v1/users>
+```
+
+The base URL has a trailing slash and the path that I want to join to it has no
+leading slash. The result is a path where `users` is joined to the end of the
+base URL. That's what I'm looking for.
+
+Now, let's see some variations on the above approach that give results that I
+wasn't expecting and don't want.
+
+```ruby
+> URI.join("https://example.com/api/v1", "/users") # 1
+=> #<URI::HTTPS https://example.com/users>
+> URI.join("https://example.com/api/v1", "users") # 2
+=> #<URI::HTTPS https://example.com/api/users>
+> URI.join("https://example.com/api/v1/", "/users") # 3
+=> #<URI::HTTPS https://example.com/users>
+```
+
+1. No trailing slash on the base URL. Leading slash on the path to join. The
+   path portion of the base URL is wiped out and `/users` is joined in.
+2. No trailing slash on the base URL. No leading slash on the path to join. The
+   `users` path replaces the last part of the path in the base URL.
+3. Both a trailing slash in the base URL and a leading slash in the path to
+   join. Same behavior as 1.
+
+I have two takeaways from this:
+- Use with caution. If I'm going to use `URI.join` for this purpose, I need to
+  be careful to only use the form in the first code block.
+- The `URI.join` method is probably meant to be primarily used to join a domain
+  (e.g. `http://example.com`) that has no path with some path segment.