title: feathers-mongodb Service.find({query: {_id}}) returns null
body: <p>I have the schemas below:</p>
  
  <p><strong>students.graphql.schema.js</strong></p>
  
  <pre><code>export default [
    `
    type StudentsWithPagination {
      total: Int
      items: [Students]
    }
  
    type Students {
      _id: String!
      name: String
      address: Addresses
    }
  `,
  ];
  </code></pre>
  
  <p><strong>addresses.graphql.schema.js</strong></p>
  
  <pre><code>export default [
        `
    type AddressesWithPagination {
      total: Int
      items: [Addresses]
    }
  
    type Addresses {
      _id: String!
      title: String
    }
  `,
  ];
  </code></pre>
  
  <p>I have created two services by running <code>feathers generate service</code> <strong>students.service.js</strong> and <strong>addresses.services.js</strong>.</p>
  
  <p>When I search addresses by <code>title</code>, I get result. However, when I search by <code>_id</code>, I get null. Something like: </p>
  
  <pre><code>const studentsResolvers = {
      Students: {
          address: student =&gt; {
              const query = {
                  _id: student.address
              }
              return Addresses.find({ query }).then(result =&gt; {
                  console.log(result)
              })
          }
      }
  }
  </code></pre>
  
  <p>The code above produces <code>null</code> though <code>student.address</code> returns the right <code>address._id</code>. I still get <code>null</code> even I hardcode <code>student.address</code> with the right <code>address._id</code></p>
  
  <p>The code above will return <code>null</code> unless I search by address title. Something like: </p>
  
  <pre><code>const query = {
     title: 'my-location'
  }
  </code></pre>
  
  <p><code>_id</code> is of type <code>String</code>, not <code>ObjectID</code>. </p>
  
  <p>What am I doing wrong?</p>