What is the _root_ field in schema.xml?
solr
Solution
The `_root_` field is needed for block-join support. See here for more detailed explanation.
You can use this when you have relationships between entities and you don't want to flatten your docs, for example, one Class doc, contains many Student docs, and you want to be able to query in a more similar way as you would do it in a traditional relational DB.
Be warned though, that you cannot get all features and flexibility of a Relational DB.
Problem
I am reading solr example schema.xml. And I found a field named `_root_`. I have never seen it before. I don't know what it does. ``` <!-- points to the root document of a block of nested documents. Required for nested document support, may be removed otherwise --> <field name="_root_" type="string" indexed="true" stored="false"/> ``` Is it a new feature of solr? What are the `nested documents`? And for what situation I should use this field? My solr version is 4.6. Thanks in advance.