# Mysql – From a backup strategy perspective, would it be efficient to use a relational DB (as MySql) and a document oriented DB (as MongoDB) in the same app

I am developing an app where I need to store an undetermined number of properties for any given entity, for example:

  Entity    || # of properties**
Apples    ||  14
Oranges   ||  4
Pears     ||  12


Using a relational DB I could set a primary key for each entity and have as many rows as there are properties, but, at the best of my knowledge, I could consider that approach only if the number of properties is fixed and known in advance, but in this case there can be rows with as few as 0 properties or as much as 99.999, so I would end up having a lot of nulls hence I was thinking on setting up a document oriented module of the APP (MongoDB) to address this and it would go as follows:

{
'Apple':{'Property1','Property2','Property3','Property4',...},
'Orange':{'Property1','Property2','Property3','Property4',...}
}


I intended to relate the two databases via a primary key but I am not sure if mixing 2 types of databases is a good idea , hence the question.