The parent field mapping is defined on a child mapping, and points to the parent type this child relates to. For example, in case of a blog type and a blog_tag type child document, the mapping for blog_tag should be:

    "blog_tag" : {
        "_parent" : {
            "type" : "blog"

The mapping is automatically stored and indexed (meaning it can be searched on using the _parent field notation).

Field data loadingedit

Contrary to other fields the fielddata loading is not lazy, but eager. The reason for this is that when this field has been enabled it is going to be used in parent/child queries, which heavily relies on field data to perform efficiently. This can already be observed during indexing after refresh either automatically or manually has been executed.