Databases Reference
In-Depth Information
As discussed in Chapter 8, the overridden
SaveChanges
method of the
UnleashedObjectContext
class calls the
BeforeSaveChanges
method for every added,
modified, or deleted entity before submitting changes to the database. This method
receives an
ObjectStateEntry
that stores both current and original property values of an
entity. It checks if the entity implements the
ISavableObject
interface, calls its
BeforeSave
method.
When your object context class inherits from the
UnleashedObjectContext
or has an
equivalent functionality, property interaction rules are automatically invoked when an
entity object is saved to the database. In other words, whenever a web page changes an
Order entity through the
NorthwindEntities
context, the
SubmittedDate
is updated auto-
matically based on the value of the
OrderStatus
property. No changes are required in the
web page itself, whether it is a custom page created for editing Order entities or the
dynamic Edit page template.
NOTE
The
ISavableObject
interface and its
BeforeSave
method work a lot like the
UPDATE
triggers in Microsoft SQL Server and other relational database management systems.
Although you could have indeed implemented this logic in a database trigger, working at
the entity level gives you more flexibility and better tools for implementing business
rules. You still have the option of using triggers as an optimization step (see
Business
Rules in the Database
later in this chapter).
Complex interaction rules might not be limited by a single entity object or even a single
entity type. For example, when an order is fulfilled, the system needs to update the
product inventory to decrement the number of items in stock by the number of items
shipped to the customer. In the Northwind sample, this means that when the
OrderStatus
property of the Order entity changes to
Fulfilled
, values of the
Quantity
property of the Order_Detail entities need to be subtracted from the
UnitsInStock
proper-
ties of the corresponding Product entities. In other words, the fulfillment business rule
spans three classes,
Order
,
Order_Detail
, and
Product
, and fits into the category of
entity
interaction rules
.
There are several different approaches you can take when implementing entity interaction
rules with Entity Framework, including (but not limited to):
.
Implementing Entity Interaction Rules as
BeforeSave
Methods
.
Implementing Entity Interaction Rules as
Entity
Methods
.
Implementing Entity Interaction Rules as
Context
Methods
