[jeremyevans]

That doesn't appear to work:

    Post.where(id: 1).or Post.joins(:author).where(author: { name: 'John' })
    ArgumentError: Relation passed to #or must be structurally compatible. Incompatible values: [:joins, :references]

It appears that only the filter clauses are allowed to be different (similar errors if :select, :order, :group, :limit are different), in which case it's no more powerful than Sequel, just more of a pain to use. You can easily implement ActiveRecord's behavior in Sequel if you want to combine filter clauses for arbitrary datasets:

    ds = Post.where(id: 2)
    Post.where(id: 1).or(ds.opts[:where])

It's also interesting what happens if you mix where and having clauses (I'm not saying it doesn't make sense, but it may bite someone):

    Post.where(id: 1).or(Post.having(id: 2)).to_sql
    # SELECT "posts".* FROM "posts"