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How should I fix this incorrect error message (don't quite understand the code)

I saw [this report about a bug in an error message](https://meta.codidact.com/posts/287036) and figured that would be an easy fix.  The incorrect error message is: "Tags can't have more than 5 tags".  That should be "Posts can't have...".  I figured I'd need to find and edit a string.

I grepped the code and found my way to `app/models/post.rb`, which includes this:

```
  def maximum_tags
    if tags_cache.length > 5
      errors.add(:tags, "can't have more than 5 tags")
    elsif tags_cache.empty?
      errors.add(:tags, 'must have at least one tag')
    end
  end
```

What is `:tags` here?  I don't see anything in this file that defines it, so it must be coming from somewhere else.  (Have I mentioned I'm very much a newbie with Ruby and Rails?)

I think the logic here is that the error is being _caused_ by tags validation, so we're adding it to a set of errors about that field.  But we don't want to use the name of the field in the error message here.

I realize it's possible to hack around this problem by changing the string here to a sentence that can legitimately begin with the word "Tags", but I'd rather understand what the correct change would be.