anchor.diff:    }
   }
   
  -PUT my_index/doc/4?routing=1&refresh <1>
  +PUT my_index/doc/4?routing=1&refresh
   {
     "text": "This is a another child document",
     "my_join_field": {
  
positive.code: 
  When indexing a child, the name of the relation as well as the parent id of the document
  must be added in the `_source`.
  It is required to index the lineage of a parent in the same shard so you must
  always route child documents using their greater parent id.
  For instance the following index two children documents pointing to the same parent
  with a `routing` value equals to the `id` of the parent:
  
  [source,js]
  
negative.code: ==== Parent-join restrictions
  
  * Only one `join` field is allowed per index mapping.
  * Parent and child documents must be indexed on the same shard.
    This means that the same `routing` value needs to be provided when
    <<docs-get,getting>>, <<docs-delete,deleting>>, or <<docs-update,updating>>
    a child document.
  * An element can have multiple children but only one parent.
  * It is possible to add a new relation to an existing `join` field.
  * It is also possible to add a child to an existing element
    but only if the element is already a parent.
  