This code block is extremely confusing. Simplify by breaking into two methods. One which determines if the @client is mongos? and another which determines the value of a read preference for a mongos (which I think should be part of the read preference classes).

```ruby
def read_pref
  read.mongos if @client.mongos?
end
```

Then in the read preference classes we can have
```ruby
class Primary
  def self.mongos
    nil
  end
end

class SecondaryPreferred
  def self.mongos
    {'readPreference' => 'secondaryPreferred', 'tagSets' => tags} if tags_set?
  end
end
```