title: Querying MongoDB to get a single embedded document that is an array element
body: <p>Good day! I am facing a challenge for getting as query result only one of multiple embedded documents. It concerns a collection of exchange rate documents, one per currency. USD is shown below. Each currency document contains multiple data points (documents that are elements of the Rate list). The purpose is to get, for instance, "all key/value pairs of the embedded document having most recent EuroRate value of type "Business" concerning currency "USD". In this case, from the example below I would like to get the full embedded document having _id: ObjectId("5e6ad7e060d2b6ccb5be2992"). 
  What would be the query instruction for this? Note: ultimately I will implement the query through Phython.</p>
  
  <pre><code>{
      _id: ObjectId("5e6ad7cb60d2b6ccb5be2908"),
      Currency:"USD",
      Rate:[
          {
              _id: ObjectId("5e6ad7cb60d2b6ccb5be2907")
              EuroRate: 0.854847},
              Type:"Business",
              SourceDate: 2019-01-01T00:00:00.000+00:00,
              DataSource:"X-Book"
          },    
          {
              _id: ObjectId("5e6ad7e060d2b6ccb5be2992")
              EuroRate: 0.854847,
              Type:"Business",
              SourceDate: 2020-01-01T00:00:00.000+00:00,
              DataSource:"X-Book"
          }
      ]
  }
  </code></pre>