Q:

How do I write an ActiveRecord query that filters out results from a joined table?

Users and Sessions are joined by a has_and_belongs_to_many association.
How do I get the unique list of Users that meet the following conditions?

user.coach == true
user.available == true

And then NOT include a user if that user is a coach in any active Session:

session.coach_id == user.id
session.call_ends_at == nil

Is there a way I can write this with ActiveRecord Query language?  Do I need to write a pure SQL statement?  Some kind of hybrid?  What would YOU do?
I also have scopes defined that could be helpful here.  But I'm not sure how to add them in:

User.available_coaches (scope)
Session.in_progress (scope)

User model
class User < ActiveRecord::Base
  has_many :client_sessions, class_name: 'Session', foreign_key: :client_id
  has_many :coach_sessions, class_name: 'Session', foreign_key: :coach_id

  scope :coaches, -> { where(coach: true) }
  scope :available_coaches, -> { coaches.where(available: true) }

Session model
class Session < ActiveRecord::Base
  belongs_to :client, class_name: 'User'
  belongs_to :coach, class_name: 'User'

  scope :in_progress, -> { where.not(coach: nil).where(call_ends_at: nil) }

Schema
create_table "sessions", force: :cascade do |t|
  t.integer  "client_id"
  t.integer  "coach_id"
  t.boolean  "canceled",          default: false
  t.datetime "coach_accepted_at"
  t.datetime "call_begins_at"
  t.datetime "call_ends_at"
  t.datetime "created_at",                        null: false
  t.datetime "updated_at",                        null: false
end

add_index "sessions", ["client_id"], name: "index_sessions_on_client_id", using: :btree
add_index "sessions", ["coach_id"], name: "index_sessions_on_coach_id", using: :btree

create_table "users", force: :cascade do |t|
  t.string   "first_name"
  t.string   "last_name"
  t.boolean  "coach",             default: false
  t.boolean  "available",         default: false
  t.datetime "created_at",                        null: false
  t.datetime "updated_at",                        null: false
end

A:

I would do it with SQL exists:
User.where(coach: true, available: true).
  where("not exists (select 1 from sessions " +
    "where sessions.coach_id = users.id and sessions.call_ends_at is null)")

Note that since there is no join to sessions there is no need for .uniq.

