Enity FrameWork missing property attributes
I created a Model using EE 4Model designer(Lets say it has Entities as EntityA, EntityB, EntityC). Built the Error 1 Error 5: The required attribute 'Relationship' is missing. to MVC framework, may be i am missing a link here. They're the result of underlying Entity Framework (EF) behavior and, infer various relationships with varying combinations of properties in your classes. NET Framework 4 -- which don't require a foreign key property in the. If the relationship is optional you have a clear indication that there is no child by setting the property to null. In case you have only childId, you.
Notice that it's non-nullable. That's because the Leaf.
Creating a Database-First Model in Entity Framework - Exception Not Found
Photo property is required see Figure 1. Your app might let you create trees before the photos have been taken, but the tree still needs that Photo property to be populated.
I'll add logic into the Tree repository's InsertOrUpdate method to create a default, empty Photo for new Trees when one isn't supplied: The bigger problem I want to focus on here is how this issue affects updates. Imagine you have a Tree and its required Photo already stored in the database. You want to be able to edit a Tree and have no need to interact with the Photo. You'll retrieve the Tree, perhaps with code such as "context.
But Tree has a photo! It's in the database! When you first execute a query to retrieve the table, ignoring the related Photo, only the scalar values of the Tree will be returned from the database and Photo will be null see Figure 2. A Tree instance retrieved from the database without its photo. When it's time to save the edited Tree, its Photo will still be null.
IsValid check in the controller code, it will recognize that Photo is missing. IsValid will be false and the controller won't even bother calling the repository. In my app, I've removed the Model Binder validation so I can let my repository code be responsible for any server-side validation. When the repository calls SaveChanges, EF validation will detect the missing Photo and throw an exception.
But in the repository, we have an opportunity to handle the problem. If the Tree class had a foreign key property -- for example, int PhotoId -- that was required allowing you to remove the requirement on the Photo navigation propertythe foreign key value from the database would've been used to populate the PhotoId property of the Tree instance. The tree would be valid, and SaveChanges would be able to send the Update command to the database.
- Using the code
- Required relationship
In other words, if there were a foreign key property, the Tree would have been valid even without the Photo instance. But without the foreign key, you'll again need some mechanism for providing the Photo before saving changes. If you have your Code First classes and context set up to perform lazy loading, any mention of Photo in your code will cause EF to load the instance from the database.
I'm still somewhat old-fashioned when it comes to lazy loading, so my personal choice would probably be to perform an explicit load from the database.
EF Code First: Add a Foreign Key relationship
The new line of code the last line in the following example, where I'm calling Load uses the DbContext method for loading related data: Tree will validate because Photo is there, and EF will send an Update to the database for the modified Tree.
The key here is that you need to ensure the Photo isn't null; I've shown you one way to satisfy that constraint. But the EF Context can't decide itself if it can drop the database to recreate it. There are two solutions: You can delete the database yourself and rerun the application.
Exception Not Found
I won't explain the first solution. About the second one, it's a feature allowed by Code First. You can ask the context to automatically drop and recreate the database if the model changes. You should be very careful with this option, because in a production environment, you can lose all your data. Because we don't want to drop the database ourselves, we will see how to tell the context to do it for us.
Manually Creating the Navigation Properties using Model Designer. | The catchsomeair.us Forums
In the file MyContext. You can do more things in the initializer but we will see that later in this post. Now we will set the initializer. Go back to Program.Fluent API c# entity framework / complextype
Further, it appears to have created a virtual property for Products. But what does that mean? Types of Relationships In relational databases, tables are related to each other via the use of keys.
One table has a "foreign key" to another table, and the kind of relationship tells you something about how objects in each of those tables are related. For example, in our Northwind database we have the tables Region and Territory. That relationship is shown in this snippet from the full Northwind diagram: Those signify the relationship between the two tables: This is called a 1-to-many relationship.
We should also look at the relationship between Category and Product: This relationship looks similar, but now we have This is called a zero-or-one-to-many relationship, and is merely a construct of having the foreign key column in the related table be nullable.
In other words, a Product can have a related Category, but it doesn't have to, and any given Category may have multiple Products associated to it.
Finally let's take a look at the Customer and CustomerDemographic tables. Here's another snippet from the main diagram: This is called a many-to-many relationship. It means that one Customer could have many CustomerDemographic entries, and one CustomerDemographic could have many Customers. In a database, a many-to-many relationship requires the existence of another tablecalled a mapping table or relationship table, that shows which items from A the left side of the relationship are related to which items from B the right side.
But Entity Framework is smart, and it realizes that this mapping table is actually not an entity unto itself, but instead is simply a relationship between two existing entities.