title: How to filter in python eve by object id string without schema
body: <p>I am using python eve to build a data storage service. All my collections are schemaless, so I've set <code>ALLOW_UNKNOWN</code> setting to True. Everything works fine except with fields referencing another ones. I'm trying to request a collection filtering by an object id string and I am always getting an empty list.</p>
  
  <p>For example, I have <code>apps</code> and <code>task</code> collections set in <code>DOMAIN</code> setting.</p>
  
  <p>The sequence of request I do is as follow:</p>
  
  <p>POST '/apps':</p>
  
  <pre><code>{
      "name": "test"
  }
  </code></pre>
  
  <p>GET '/apps':</p>
  
  <pre><code>{
      "_items": [
          {
              "_id": "5afb45872fb8d57c7d93d8fa",
              "name": "Test app",
              "_updated": "Tue, 15 May 2018 20:39:35 GMT",
              "_created": "Tue, 15 May 2018 20:39:35 GMT"
          }
      ],
      "_meta": {
          "page": 1,
          "max_results": 25,
          "total": 1
      }
  }
  </code></pre>
  
  <p>POST '/tasks':</p>
  
  <pre><code>{
      "title": "Check log files",
      "app_id": "5afb45872fb8d57c7d93d8fa"
  }
  </code></pre>
  
  <p>GET '/tasks':</p>
  
  <pre><code>{
      "_items": [
          {
              "_id": "5afb4bcd2fb8d57c7d93d912",
              "title": "Check log files",
              "app_id": "5afb45872fb8d57c7d93d8fa",
              "_updated": "Tue, 15 May 2018 21:06:21 GMT",
              "_created": "Tue, 15 May 2018 21:06:21 GMT"
          }
      ],
      "_meta": {
          "page": 1,
          "max_results": 25,
          "total": 1
      }
  }
  </code></pre>
  
  <p>GET '/tasks/?where={"app_id": "5afb45872fb8d57c7d93d8fa"}:</p>
  
  <pre><code>{
      "_items": [],
      "_meta": {
          "page": 1,
          "max_results": 25,
          "total": 0
      }
  }
  </code></pre>
  
  <p>My question are:</p>
  
  <ul>
  <li>How can I post an ObjectId field without a defined schema?</li>
  <li>How can I retrieve tasks filtering by <code>app_id</code>?</li>
  </ul>
  
  <p>Looking at docs I've found the <code>query_objectid_as_string</code> setting, however document endpoints return 404 error.</p>
  
  <p>GET /tasks/5afb4bcd2fb8d57c7d93d912 -> Not found</p>