Question

What are the pros and cons of using embedded documents when you can simply use an array datatype? Both seem similar to me (and I couldn’t find any information online via google search). Please provide example cases!

Answer 1

In terms of data structures, you can think of embedded documents as hashes or dictionaries .. while arrays are a list of values.

With embedded documents in MongoDB:

• embedded documents have named fields, and can embed other documents for rich data representation
• you can reference fields directly using dotted notation
• creating an index on an embedded document field only indexes that field
• you can use field selection to retrieve a subset of fields.

With arrays in MongoDB:

• you can manipulate arrays using operators such as $push, $pop, $pull, and $addToSet.
• you can match array values using operators such as $all, $in, $nin.
• you can also use multikey indexes
• creating an index on an array element indexes each element of the array.
• you can use the $slice operator to retrieve a subset of an array.

Mongoid’s notion of relations express a few different combinations of embedded documents and arrays:

• embeds_one – a single embedded document
• embeds_many – an array of embedded documents