`self.class` here evaluates to `Class` and then we're calling `define_method` on it which means all classes will have those methods defined in them. For example:
```
~/discourse(master*) » rails c
Loading development environment (Rails 6.0.3.3)
[1] pry(main)> Integer.methods
=> [:sqrt,
:yaml_tag,
:email_domains_blacklist=,
:email_domains_whitelist=,
:unicode_username_character_whitelist=,
:user_website_domains_whitelist=,
:whitelisted_link_domains=,
:email_domains_blacklist,
:email_domains_whitelist,
:unicode_username_character_whitelist,
...
...
```
Fix here is to use `self.define_singleton_method`.
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