title: Set a custom field as argument for .find() in mongoose
body: <p>i'm doing an HTTP call to my api to retrieve documents from a mongodb and in the url i have two parameters: <strong>campo</strong> and <strong>keyphrase</strong>.
  I wanna look for all the documents which have the parameter <strong>campo</strong> set to my <strong>keyphrase</strong>.
  E.g: I have a few docs like this in my mongo: </p>
  
  <pre><code>classe: "4AT"
  cognome: "XYZ"
  email: "xyz.zyx@gmail.com"
  idreg: "IUDQFEXFFR4DW133"
  nome: "Francesco"
  turni: ["g2t1", "g1t2", "g3t2"]
  __v: 0
  _id: "5c508d0b4810fc0ece8df2a9"
  </code></pre>
  
  <p>I call GET to "localhost:3000/nome/Francesco" and i wanna pick all the documents with value "Francesco" in field "nome".
  But the problem is that the method .find() accepts parameters like:</p>
  
  <pre><code>.find({nome: keyphrase})
  </code></pre>
  
  <p>and not dynamical headers like i want. Is there any chance to do it?</p>
  
  <p>PS: my Express code for get request, if it helps:</p>
  
  <pre><code>app.get("/api/ragazzi/:campo/:keyphrase",(req, res, next) =&gt; {
    let campo = req.params.campo.toLowerCase();
    let keyphrase = req.params.keyphrase;
    Ragazzo.find({campo : keyphrase})
      .then(documents =&gt;{
        res.status(200).json({
          codice: "ok",
          risultato: documents
        });
      })
      .catch((err)=&gt;{console.log(err);});
  });
  </code></pre>