ActiveRecord question - how to find owners of at least one child with a conditional

Hopefully the subject is clear enough.

Let's say I've got two tables:

create_table :bands do | b |
    b.column :name, :string
end

create_table :concerts do | c |
  c.column :where, :string
  c.column :when, :date
  c.column :band_id, :integer
end

and the models:

class Band <<ActiveRecord::Base
  has_many :concerts
end

class Concert << ActiveRecord::Base
  belongs_to :band
end

What I'd like to do is find all of the Bands which have at least one
concert on or after a given date.

In this case there are lots of concerts which makes this a bit
performance critical.

I've not been able to figure out how to do this through ActiveRecord
so as to generate an efficient SQL query, or how to do it manually
using find_by_sql either. It's a bit complicated also since SQL date
comparison function seems to be somewhat database dependent.

Any ideas from the community?