Struggling with a complex Rails 3 query (user.friends.checkins)

I'm working on a social networking application and am trying to build
a complex query that efficiently pulls all of a users' friends'
checkins from the database. Basically I need: "user.friends.checkins"

I've recreated (in simplified form) the data structure below for
reference and included several solutions I've found, however I feel
like my solutions are sub-optimal. I'm hoping that someone can point
out a better way... So here goes:

# === My Solutions ===
I tried this first, which works, but is way too slow given that users
typically have +1000 friends each:
=> Checkin.where(:user_id => self.friends)

Then I tried this, which also works and is much faster, but feels
sloppy:
=> Checkin.joins(:user).joins('INNER JOIN "friendships" ON
"users"."id" = "friendships"."friend_id"').where(:friendships =>
{:user_id => 1})

Any help you can provide would be GREATLY appreciated! Thanks in
advance!!!

# === Data structure ===
# Users table columns: id, name
# Friendships table columns: user_id, friend_id
# Checkin table columns: id, location, user_id

# === Models ===
class User
  has_many :friendships
  has_many :friends, :through => :friendships
  has_many :checkins
end

class Friendship
  belongs_to :user
  belongs_to :friend, :class_name => 'User'
end

class Checkin
  belongs_to :user
end