title: How to create a join query between two entities in FIWARE orion?
body: <p>I have two entites A (postings) and B (users) with a many to one relationship and I want to do an SQL like join to list posts and users together.
  I tried the following query, but it only returns an empty array.
  I could not find any other documentation regarding this, all tutorials only refer to one id.</p>
  
  <pre><code>https://{{orion}}/v2/entities?q=Post.createdBy==User.id&amp;options=count,keyValues&amp;limit=100
  </code></pre>
  
  <p>I also tried this variant, but the same result:</p>
  
  <pre><code>https://{{orion}}/v2/entities?q=createdBy==id&amp;options=count,keyValues&amp;limit=100
  </code></pre>
  
  <p>And I checked that there is at least one entity which matches I manually checked</p>
  
  <pre><code>https://{{orion}}/v2/entities?q=createdBy==urn:ngsi-ld:User:78bc5cac26d1b3abe27cb2cf94b1015a3710958f 
  </code></pre>
  
  <p>This query above returns an entity.</p>
  
  <pre><code>|      Post        |     |     User     |
  |------------------|     |--------------|
  | ID               |     | ID           |
  | Type             |     | Type         |
  | Message          |     | Username     |
  | CreatedBy: UserId|
  </code></pre>
  
  <p>In orion the datatype:</p>
  
  <pre><code>   "createdBy": {
              "type": "Relationship",
              "value": "urn:ngsi-ld:User:78bc5cac26d1b3abe27cb2cf94b1015a3710958f",
              "metadata": {}
          },
  </code></pre>