Model Relationships - CA
The way we experienced relationships in childhood creates an We can change our model, but we have to identify it, so we can challenge it. A non-identifying relationship is when the primary key attributes of the parent must . you can remove the author (FK) from the book row and still can identify the. Figure The Identify Relationships Process: ReConnect analyzes col- learning algorithms on training data to recognize patterns. SVMs are Input: a vector of binary features Fij = [f0,,fk], where f0, f1,, fk are cell.
The following code listing shows a one-to-many relationship between Blog and Post Post is the dependent entity Blog is the principal entity Post. BlogId is the foreign key Blog. BlogId is the principal key in this case it is a primary key rather than an alternate key Post.Entity Relationship Diagram (ERD) Tutorial - Part 1
Blog is a reference navigation property Blog. Posts is a collection navigation property Post. Blog is the inverse navigation property of Blog. A property is considered a navigation property if the type it points to can not be mapped as a scalar type by the current database provider. Note Relationships that are discovered by convention will always target the primary key of the principal entity.
To target an alternate key, additional configuration must be performed using the Fluent API. Fully Defined Relationships The most common pattern for relationships is to have navigation properties defined on both ends of the relationship and a foreign key property defined in the dependent entity class. If a pair of navigation properties is found between two types, then they will be configured as inverse navigation properties of the same relationship.
No Foreign Key Property While it is recommended to have a foreign key property defined in the dependent entity class, it is not required. You can also have a single navigation property and a foreign key property. Cascade means dependent entities are also deleted.
ClientSetNull means that dependent entities that are not loaded into memory will remain unchanged and must be manually deleted, or updated to point to a valid principal entity. For entities that are loaded into memory, EF Core will attempt to set the foreign key properties to null. See the Required and Optional Relationships section for the difference between required and optional relationships. See Cascade Delete for more details about the different delete behaviors and the defaults used by convention.
Data Annotations There are two data annotations that can be used to configure relationships, [ForeignKey] and [InverseProperty].
Relationships - EF Core | Microsoft Docs
This is typically done when the foreign key property is not discovered by convention. It does not need to go on the navigation property in the dependent entity class. This is typically done when there is more than one pair of navigation properties between two entity types. HasOne or HasMany identifies the navigation property on the entity type you are beginning the configuration on. You then chain a call to WithOne or WithMany to identify the inverse navigation.
This indicates that there is conceptually a reference or collection on the other end of the relationship, but there is no navigation property included in the entity class.
This rule appears as I: R placed next to the child entity in the relationship. This means that an instance can be added to the child entity only if the referenced foreign key matches an existing instance in the parent entity.
In this type of relationship, the referential integrity options are very different: The Delete does not cascade as in our previous example, and it is not prohibited as in Restrict. Use of Cascade or Set Null should reflect business decisions about maintaining the historical knowledge of relationships, represented by the foreign keys.
These relationship exceptions include: N-ary relationships A simple one-to-many relationship between two entities is termed binary. When a one-to-many relationship exists between two or more parents and a single child entity, it is termed an n-ary relationship.
Recursive relationships Entities that have a relationship to themselves take part in recursive relationships. This type of relationship is also used for bill-of-materials structures, to show relationships between parts. Subtype relationships Related entities are grouped together so that all common attributes appear in a single entity, but all attributes that are not in common appear in separate, related entities.
Many-to-Many Relationships In key-based and fully-attributed models, relationships must relate zero or one instances in a parent entity to a specific set of instances in a child entity. As a result of this rule, many-to-many relationships that were discovered and documented in an ERD or earlier modeling phase must be broken down into a pair of one-to-many relationships. You can eliminate a many-to-many relationship by creating an associative entity.
Many-to-many relationships often hide meaning. When you resolve the many-to-many relationship, you see not only how the entities are related, but uncover additional information, such as the "course-time," which also describes facts about the relationship.
Once the many-to-many relationship is resolved, you are faced with the requirement to include relationship verb phrases that validate the structure. There are two ways to do this: The most straightforward way is to continue to read the many-to-many relationship, through the associative entity. Many modelers adopt this style for constructing and reading a model.
There is another style, which is equally correct, but a bit more cumbersome.
Identifying and Non-Identifying Relationships
The structure of the model is exactly the same, but the verb phrases are different, and the model is read in a slightly different way: Although the verb phrases are now quite long, the reading follows the standard pattern; reading directly from the parent entity to the child.
Whichever style you choose, be consistent. Deciding how to record verb phrases for many-to-many relationships is not too difficult when the structures are fairly simple, as in these examples.
However, this can become more difficult when the structures become more complex, such as when the entities on either side of the associative entities are themselves associative entities, which are there to represent other many-to-many relationships.