I am new to Entity Framework. When the Visual Studio creates Model diagram we can see mainly two things in Entities.Propertie and Navigation Properties,So what are these Navigation Properties? How to use them?
Navigation properties represents related entites to the principal entity. Foreign Keys are usually represented by navigation properties.
Ex : if you have two tables Invoice and invoice items and those tables have a relation 1-> many so you'll find a navigation property in invoice entity that lists all invoice items related to an invoice.
Hope it helps.
Navigation properties in the Entity Framework provide a way to navigate an association between two entity types. Every object can have a navigation property for every relationship in which it participates. Navigation properties allow you to navigate and manage relationships in both directions, returning either an EntityReference, if the multiplicity is either one or zero-or-one, or an EntityCollection, if the multiplicity is many.
When you use the Entity Framework-generated classes, navigation properties are created for objects that participate in a relationship.
UPDATE: Here is nice navigation properties example for relations between books, authors and publishers.
Navigation Property is mainly used for Foreign key relationship in EF. i.e. User to Roles, product to categories etc.
so if you have Order with OrderLines, navigation property will say Order_OrderLineItems and you can access complete line items associated with it.
have a look some of the explanation here, What are Navigation Properties in Entity Framework for?
Related
I am using an Entity Framework 6 ,I have some table as a mapping tables in the database - to map many to many tables - does not created as an entity when I create the model. what is the problem will be?
that's the tables:
Prop:
PK-Code
PropCh:
PK-CHID
and the mapping table:
Ch_Prop:
PK-CHID
PK-Code
In your schema, the mapping table has no properties other than the FKs of the related entities, what makes it so to say a pure join table.
For this reason, EF does not creates a class to represent that "model", as entries of the table will be internally represented by the state of the navigation properties among the related models.
In most cases this isnt an issue, as normally entities are pre-loaded before being associated through modifications of their navigation properties.
Having two tables Courses and Employees, i have a many to many relationship between the two. EF automatically generates an intermediary table called CoursesEmployees.
I want to add a property IsPassed to that intermediary.
I could do this in MySQL but i won't know where to map it to in my data model. Is it possible? Preferably in the data model because i work model-first.
I use all of the latest versions.
The way to achieve what you want in a Model First approach is to explicitly model the relation table and add your IsPassedproperty to it, as I've done below. There is a 1-to-m relationship between Course and Course_Employee entities as well as a 1-to-n relationship between Employee and Course_Employee entities. The combination of the two one-to-many relationships creates the many-to-many relationship between Course and Employee entities.
I am working on an MVC project in Visual Studio Ultimate 2013 and I have some unwanted columns in some of my tables and I cannot really understand why is it so.
In table Categories there is the column Template_TemplateId which is probably because in the Template entity class there is a ICollection<Category> property.
I want a Template to have many categories, however a one category can be repeatedly used by many Templates so I guess the Template_TemplateId column is something I don't want there.
Can someone explain, why is this so, and what should be a right approach to have the model the way I want it?
Thank you.
Model First:
Be default, EntityFramework creates one-to-many type relationships. When you create those, you need a foreign key in the "one" side of the relationship.
This is the "Template_TemplateID" field. It is the foreign key into the "Templates" category. However, you said you wanted a many-to-many relationship, so you are absolutely right; that field is useless!
You need to specify a cardinality of "*" on both ends of the relationship, then the model will create an intermediate table allowing the many-to-many relationship. The foreign key should go away at this point.
Code First:
Same problem, but the solution is to make a collection of "Template" in categories and and a collection of "Category" in templates, thus creating the many-to-many relationship.
If you want a many-to-many relationship between Templates and Categories create two collections, on on each class - on Template, create a ICollection and on Category, create ICollection.
I noticed something when i was reading through some Entity Framework's POCO classes that the one to many relationship is always represented in two ways like following :
1- Public List<User> Users {get;set;}
2- Public Virtual User Users {get;set}
So which one is right and when i should use each, this concept really confused me !!!
I think you've read that wrong. Typically (although not required), you would have navigation properties at both ends of the relationship.
A collection navigation property on the one side (a Department may have a List<User> for example) and a reference navigation property on the many side (a User would have one Department).
It is also recommended that you have a foreign key property as well, for example an int DepartmentId on the User.
You would need to mark the navigation properties as virtual if you wanted to support lazy loading. See here for the requirements on POCO types.
I would like to have entity which can have a child (one or zero). This child is same type as parent. I am not sure how to set entity framework becouse I would like to have two navigation properties for every entity. One for navigation to child and one for navigation to parent. Basically it is exactly the same structure as doubly linked list.
I think this table structure should be enough:
int | id | PK
int | id_next | FK
text | data
But how can I create navigation properties for next/prev items? I am able to create only navigation property for next item.
Thanks for help.
You can't. The problem here is that a one-to-one relation has a very specific requirement - FK value must be unique in the whole table. Once the uniqueness is not enforced you can add a second entity pointing to the same parent and you have a one-to-many relation.
To enforce this in a self referencing relation like you described in your example you will place an unique index on the id_next and it will work in SQL server. The problem is that entity framework doesn't support unique keys. Because of that entity framework is only able to build one-to-one relations between two different entity types where FK in the dependent the entity type is also its PK (the only way how to force FK to be unique) = both entities has same PK value. This cannot work with a self referencing relation because you cannot have two same PK values in one table.
You can do this in EF4 by specifying a 0..1 -> 0..1 relationship on the entity. Name one of the navigation properties "Previous" and the other "Next". This will create a hidden field on the underlying DB.
I haven't thoroughly tested this approach but it seemed to work when I created the database script.
Research Tree structures in the Entity Framework. You basically want a vertical tree (i.e. one branch). The framework won't enforce only one branch, but you can manage that in your business logic.