Mongo DB does not support joins or multidocument transactions at all, MongoDB has been able to implement an automatic sharding solution with much better scaling and performance characteristics than you’d normally be stuck with if you had to take relational joins and transactions into account.
Embedding data vs Referencing
Embedded data –
We can say embedded data is denormalized form, embedding is the approach that will provide the best performance and data consistency guarantees. Example of a blog schema-
{
"_id": "First Post",
"author": "Rick",
"text": "This is my first post",
"comments": [
{ "author": "Stuart", "text": "Nice post!" },
...
]
}
Referencing Data
If your application may query data in many different ways, or you are not able to anticipate the patterns in which data may be queried, a more “normalized” approach may be better. Normalizing your data model into multiple collections is the increased flexibility this gives you in performing queries.
// db.posts schema
{
"_id": "First Post",
"author": "Rick",
"text": "This is my first post"
}
// db.comments schema
{
"_id": ObjectId(...),
"post_id": "First Post",
"author": "Stuart",
"text": "Nice post!"
}
MongoDB tends to be more of an art than a science, and one of the earlier decisions you need to make is whether to embed a one-to-many relationship as an array of subdocuments or whether to follow a more relational approach and reference documents by their _id value.
Roller Coaster of Technology, Design and Life 