title: performing priority query in mongo
body: <p>sample document :</p>
  
  <pre><code>{"name":"John", "age":35, "address":".....",.....}
  </code></pre>
  
  <ol>
  <li>Employees whose join_month=3 is priority 1 </li>
  <li>Employees whose address contains the string "Avenue" is priority 2 </li>
  <li>Employees whose address contains the string "Street" is priority 3 </li>
  <li>Employees whose address contains the string "Road" is priority 4 </li>
  </ol>
  
  <p>As of now, I'm at this stage:</p>
  
  <pre><code>db.collection.aggregate([
      { "$match": { 
          "$or": [ 
              { "join_month": 3 }, 
              { "address": /.*Avenue.*/i }, 
              { "address": /.*Street.*/i }, 
              { "address": /.*Road.*/i }
          ] 
      }}, 
      { "$project": { 
          "name": 1, 
          "age": 1,
          "_id": 0, 
          "priority": { ?????????? } 
      }}, 
      { "$sort":  { "priority": 1 } }
  ])
  </code></pre>
  
  <p>I'm stuck at priority field. What should I put there?</p>