Database Reference
In-Depth Information
On the model in Example 1, we see that
Account Number
is unique, and therefore to re-
trieve a specific
Account
requires only knowing the
Account Number
and not anything
from
Customer
, making
Account
an independent entity because
Account Number
be-
longs to
Account
. However, in Example 2,
Account Number
34
appears three times, so
the only way to find a specific account is to know who the customer is. So the combina-
tion of
Customer ID
and
Account Number
is required to distinguish a particular account,
making
Account
in Example 2 a dependent entity because
Customer ID
, which is needed
to help identify accounts, does not belong to
Account
.
Notice also that the relationship line looks different when connecting to an independent
verses dependent entity. The dotted line means
non-identifying
and the solid line means
identifying
. Identifying relationships mean that the entity on the many side (the child) is
always going to be a dependent entity (like Account in Example 2) to the entity on the one
side (the parent).
If we are designing a MongoDB database and trying to decide whether we should embed
documents or reference other documents, a major factor would be whether the relationship
is identifying or non-identifying. An identifying relationship is a much stronger relation-
ship than non-identifying, giving us more weight towards embedding the weak entity with-
in the strong entity so that we have just one document.
1
There is a one-to-one relationship between two entities
. A one-to-one relation-
ship means that for a given entity instance, there can be at most one entity in-
stance relating to it, meaning if we decide to embed, there will be no redund-
