title: Mongoose delete nested subdocuments and documents
body: <p>I have:</p>
  
  <pre><code>    let userSchema = mongoose.Schema({
  email: {type: String, required: true, unique: true},
  passwordHash: {type: String, required: true},
  fullName: {type: String, required: true},
  salt: {type: String, required: true},
  ads: [{type: ObjectId, ref: 'Ad'}],
  roles: [{type: String}]
  </code></pre>
  
  <p>}</p>
  
  <pre><code>let adSchema = mongoose.Schema({
  author: {type: ObjectId, ref: 'User'},
  title: {type: String, required: true},
  category: {type: ObjectId, ref: 'Category', required: true},
  town: {type: ObjectId, ref: 'Town', required: true},
  </code></pre>
  
  <p>}
  );</p>
  
  <pre><code>let categorySchema = mongoose.Schema({
  name: {type: String, required: true, unique: true},
  ads: [{type: ObjectId, ref: 'Ad'}]
  </code></pre>
  
  <p>}
  );</p>
  
  <pre><code>let townSchema = mongoose.Schema({
  name: {type: String, required: true, unique: true},
  ads: [{type: ObjectId, ref: 'Ad'}]
  </code></pre>
  
  <p>}
  );</p>
  
  <p>I want to find for example town by id and remove all ads in it(and ofcourse to remove the ads from their categories and authors).How can i do that?</p>