Database Reference
In-Depth Information
SM385932
1
5/10/2013
EM584926
1
5/10/2013
EM584926
2
6/10/2013
MM481526
2
6/10/2013
MM573483
2
6/10/2013
LR731511
3
7/10/2013
By looking at these values and recalling the students referenced by the
Student Number
and classes reference by
Class ID
, we learn that
Steve Martin
and
Eddie Murphy
both at-
tended the
Data Modeling Fundamentals
class on
5/10/2013
.
Eddie Murphy
also attended
the
Advanced Data Modeling Class
with
Mickey
and
Minnie Mouse
on
6/10/2013
.
Lone
Ranger
took
Tennis Basics
(by himself, as usual) on
7/10/2013
.
MongoDB Reference = RDBMS Foreign Key
The concept of a foreign key in relational databases is similar to the concept of a reference
in MongoDB. Both a foreign key and a reference provide a way to navigate to another
structure. The difference, though, is that the relational database automatically ensures each
foreign key value also exists as a value in the originating primary key. That is, if there is a
Product ID
foreign key in
Order Line
originating from
Product
, every
Product ID
value
in
Order Line
must also exist in
Product
. This check of valid values from foreign keys
back to their primary keys is called “referential integrity” (or RI for short). In MongoDB,
the reference is simply a way to navigate to another structure (without the RI).
For example, recall our data model:
