bind.js

function _promiseForAssessmentPastDue (assessment, studentSolution, stepId) {
  // use Promise resolve so that we can .spread later on (.spread is not a method on the mongoose object)
  return Promise.resolve(mongoose.model('User').findById(studentSolution.user))
    .then(_user => {
      return Promise.all([
        mongoose.model('CohortAssessment').findOne({ assessment: assessment._id, cohort: _user.cohort }), 
        mongoose.model('Solution').find({ user: studentSolution.user, step: stepId })
      ])  
    })
    .spread(function(cohortAssessment, gitHubSolution) {     
      let now = Date.now();
      return (_pastEndDate(now, cohortAssessment.end) || _pastDuration(gitHubSolution.start, now, cohortAssessment.duration));
    });
}


// to be clear, here's how we use bind: 

function _promiseForAssessmentPastDue (assessment, studentSolution, stepId) {
  // use Promise resolve so that we can .spread later on (.spread is not a method on the mongoose object)
  return Promise.resolve(mongoose.model('User').findById(studentSolution.user)).bind(this)
    .then(_user => {
      // attach info to "this", which is the promise object and can be accessed within any subsequent "then"
      this.cohort = _user.cohort; 
      return Promise.all([
        mongoose.model('CohortAssessment').findOne({ assessment: assessment._id, cohort: _user.cohort }), 
        mongoose.model('Solution').find({ user: studentSolution.user, step: stepId })
      ])  
    })
    .spread(function(cohortAssessment, gitHubSolution) {
      // we can access this.cohort down here 
      //typeof this.cohort === 'object' // true 
      let now = Date.now();
      return (_pastEndDate(now, cohortAssessment.end) || _pastDuration(gitHubSolution.start, now, cohortAssessment.duration));
    });
}