gistfile1.txt

db.foo.insertMany([
  { email: 'wes@example.com', course: 'a', purchaseDate: 10 },
  { email: 'wes@example.com', course: 'b', purchaseDate: 20 },
  { email: 'wes@example.com', course: 'c', purchaseDate: 5 },
  { email: 'nancy@example.com', course: 'c', purchaseDate: 6 },
  { email: 'nancy@example.com', course: 'b', purchaseDate: 10 },
  { email: 'nancy@example.com', course: 'a', purchaseDate: 5 },
  { email: 'nancy@example.com', course: 'd', purchaseDate: 1 },
  { email: 'john@example.com', course: 'd', purchaseDate: 1 },
  { email: 'bob@example.com', course: 'a', purchaseDate: 1 }
])

db.foo.aggregate([
  // Project the target purchasedate
  {'$project': {
    'email': 1,
    'course': 1,
    'purchaseDate': 1,
    'target': { '$cond': [{'$eq': [ "$course", "a" ]}, "$purchaseDate", 0] }
  }},
  // Group records by email, storing the target and the highest encountered purchase date
  {'$group': {
    '_id': '$email',
    'target': {'$max': '$target'},
    'max': {'$max': "$purchaseDate"}
  }},
  // Check if the highest encountered date is greater then the target
  {'$project': {
    'email': 1,
    'target':1,
    'max': 1,
    'selected': {'$cond': [{'$gt': ['$max', '$target']}, true, false] }
  }},
  // Filter out the non-matches
  {'$match': {
    'target': {'$gt': 0},
    'selected': true
  }},
])