When to use array or not to use them in mongodb -

i working first application in symfony2/mongodb, have store articles , these articles have tags, keywords , related images. @ moment storing these information that:

"category" : [     "category1",     " category2",     " category3" ], 

but saw few examples saying

"category" : "category1, category2, category3", 

so guessing 1 best way it?

if use array, have following advantages:

• you can access each item directly index.
• you can perform queries directly on array using operators $in, $nin , $elemmatch

if use string, have to:

• split , in order looping
• user text based searching in query, slow

one thing need keep in mind regarding arrays inside mongodb document should not large. arrays can large, , if pushes size of document beyond 16 mb, cause issues, 16 mb maximum allowed size single document.

in use case, can split off contents of array separate collection , created references.