mongodb schema design naming convention

I use Google's JSON Style Guide myself and they suggest using camelCase i.e. firstPostDate. Below is the section excerpt

Property Name Format

Property names must conform to the following guidelines:

• Property names should be meaningful names with defined semantics.
• Property names must be camel-cased, ascii strings.
• The first character must be a letter, an underscore (_) or a dollar sign ($).
• Subsequent characters can be a letter, a digit, an underscore, or a dollar sign.
• Reserved JavaScript keywords should be avoided (A list of reserved JavaScript keywords can be found below).

These guidelines mirror the guidelines for naming JavaScript identifiers. This allows JavaScript clients to access properties using dot notation. (for example, result.thisIsAnInstanceVariable). Here's an example of an object with one property:

{
  "thisPropertyIsAnIdentifier": "identifier value"
}

Shortening field names is not necessary.

Check docs:

Shortening field names reduces expressiveness and does not provide considerable benefit for larger documents and where document overhead is not of significant concern. Shorter field names do not reduce the size of indexes, because indexes have a predefined structure. In general it is not necessary to use short field names.