fact-document-field-name-restrictions.rst

:doc:`Documents </core/document>` have the following restrictions on field names:

• The field name _id is reserved for use as a primary key; its value must be unique in the collection, is immutable, and may be of any type other than an array.
• The field names cannot start with the dollar sign ($) character.
• The field names cannot contain the dot (.) character.
• The field names cannot contain the null character.