This is a overly simplistic sign in user action that demonstrates roughly how a sign in might work in Obvious with users and user sessions

sign_in_user.rb

require_relative '../entities/user'
require_relative '../entities/user_session'

class SignInUser

  def initialize user_jack, user_session_jack
    @user_jack = user_jack
    @user_session_jack = user_session_jack
  end

  def execute input
    unless input.has_shape? email: String, password: String
      raise ArgumentError.new 'invalid input format'
    end

    # find the user and check credentials
    # use: UserJack.get, User.populate
    user_data = @user_jack.find email: input[:email]
    user = User.new
    user.populate user_data
     
    # set default id and values for new UserSession entity
    session_data = { 
      id: -1,
      email: user_data[:email],
      token: generate_token
    }

    # create/populate UserSession object
    # use: UserSession.populate
    session = UserSession.new
    session.populate session_data    

    # save session to jack
    # use: UserSessionJack.save, UserSession.to_hash
    
    # return the result
    @user_session_jack.save session.to_hash    
  end

  def generate_token
    'random token!' 
  end
end