```ruby
steve = Person.find_by(name: "Steve")
david = Author.find_by(name: "David")
relation = Essay.where(writer: steve)
# Before
relation.rewhere(writer: david).to_a # => []
# After
relation.rewhere(writer: david).to_a # => [david]
```
For now `rewhere` only works for truly column names, doesn't work for
alias attributes, nested conditions, associations.
To fix that, need to build new where clause first, and then get
attribute names from new where clause.
