ActiveRecord::ConnectionNotEstablished - No connection pool for X
Please check how your sinatra app established the connection
configure :production, :development do db = URI.parse(ENV['DATABASE_URL'] || 'postgres://localhost/mydb') pool = ENV["DB_POOL"] || ENV['MAX_THREADS'] || 5 ActiveRecord::Base.establish_connection( adapter: db.scheme == 'postgres' ? 'postgresql' : db.scheme, host: db.host, username: db.user, password: db.password, database: db.path[1..-1], encoding: 'utf8', pool: pool )endMake sure you have proper settings for pool, also make sure you have a heroku config for DB_POOL or MAX_THREADS.
heroku config:set DB_POOL=5or
heroku config:set MAX_THREADS=5