Types of Relationships
Types of Relationships
Just as database tables can relate to each other, so can classes and objects. In fact, ORM translates relationships between objects into relationships between database rows.
This chapter introduces the tools needed to create meaningful relationships using ORM. Let’s consider the different types of relationships at a conceptual level. In later sections, we will learn how to implement these relationships using EntityFrameworkCore.
For the examples below, we use four classes:
Event- A class representing a coding event.
EventCategory- A class representing categories of coding events.
EventDetails- A class that encapsulates details about a single event, such as description, contact email, location, and so on.
The first two of these are familiar to you from our
CodingEvents app. The
Tag classes are new.
The simplest type of relationship is the one-to-one relationship.
This occurs when each instance of type A may be related to only one instance of type B, and vice versa.
In the following examples, arrows point in the direction of the relationship. If A points to B, then you can say that A knows about B.
While relationships in SQL are bidirectional, relationships in programming languages are unidirectional. In other words, if A knows about B, then B doesn’t necessarily know about A.
A one-to-one relationship between an
Event object and an
Event object should only have one collection of details, so it should only be related to one
Similarly, details about an event are specific to that event, so an
EventDetails object should only be related to one
The following pairs of things generally have one-to-one relationships:
People / driver's licenses
States / capital cities
iPhones / serial numbers
It is not required that each instance of type A be related to an instance of type B. For example, a person may not have a driver’s license.
One-to-Many and Many-to-One
A one-to-many relationship occurs when each instance of type A may be related to more than one instance of type B, but each instance of B can only be related to a single instance of type A.
A one-to-many relationship between
In this case, we say that A has a one-to-many relationship to B. A category can contain multiple items, therefore an
EventCategory object may be related to multiple
Event objects. But an event may only be in one category.
The following pairs of things generally have one-to-many relationships:
Birth dates / people
States / U.S. Representatives
Model numbers / iPhones
When discussing the inverse relationship, we say that B has a many-to-one relationship to A.
A many-to-one relationship between
A many-to-one relationship operates in the opposite direction of a one-to-many relationship.
The difference between the two is which side of the relationship knows about the objects on the other side.
In C# terms, this will translate into a property on one class that references the other.
Many-to-one relationships are simply the opposite direction of one-to-many. Therefore, each of the following pairs has a many-to-one relationship.
People / birth dates
U.S. Representatives / states
iPhones / model numbers
Many-to-many relationships occur when each instance of type A can be related to multiple instances of type B, and vice versa.
A many-to-many relationship between Event and Tag objects
An event can have multiple tags, and a tag may be associated with multiple events. Thus, we have a many-to-many relationship.
The following pairs of things generally have many-to-many relationships:
Books / authors
Recipes / ingredients
Actors / movies
Check Your Understanding
Match the following pairs with the appropriate relationship type:
car / manufacturer
car / title
car / driver
car / tire
True/False: Suppose two C# classes, A and B, are in a one-to-many relationship. Then class A must
contain a property for instances of B and B must have a property for instances of A.