MongoDB Objects - Data Modeling for MongoDB - page 39

The business rules in this example are:

• Each Author may write one or many Titles .

• Each Title must be written by one Author .

The small vertical line means one . (Looks like a 1, doesn't it?) The circle means zero .
(Looks like a zero too!) The zero implies optionality and does not exclude the value one ,
so in the above example an author can write just one title, too.

The triangle with a line through the middle means many . Some people call the many sym-
bol a crow's foot . Relationship lines are frequently labeled to clarify the relationship and
express the rule that the relationship represents. A data model is a communication tool, and
if you think of the entities as nouns, the relationship label is a present tense verb, so we are
just reading a sentence:

• Each Author may write one or many Titles .

Having a zero in the cardinality makes us use optional-sounding words such as may or can
when reading the relationship. Without the zero, we use mandatory-sounding terms such as
must or have to . So instead of being redundant and saying:

• Each Author may write zero , one or many Titles .

We take out the word zero because it is expressed using the word may , which implies the
zero:

• Each Author may write one or many Titles .