I have an MVVM application, EF code first.
In my model I have an Appointment entity and this entity has navigation properties which are Types such as AppointmentType; AppointmentDuration; AppointmentConsultant. These were all specified as 1-1 in initial requirements. So my Appointment model has the [Required] annotation on the Ids' of these types.
Of course 6 months after the client starting to use the system in anger they have identified that if they mark an appointment as "DidNotAttend" then they don't want to have to select the, once mandatory, types.
My question is, seeing as these types all exist in database tables, and my code has enumerations matching the Ids, would you add a new types, and enum values, for each of these with description of NotSelected, or simply amend the underlying database structure and model to make these not required/nullable integers, and write business logic in view model to say, "if AttendanceIndicator is NOT 'DidNotAttend' then check each is NOT 'NotSelected'"?
Related
Just starting out with Entity Framework (Code First) and I have to say I am having a lot of problems with it when loading SQL data that is fairly complex. For example, let's say I have the following tables which stores which animals belongs to which regions in the world and the animal are also categorized.
Table: Region
Id: integer
Name string
Table AnimalCategory
Id integer
Name: string
RegionId: integer -- Refers back Region
Table Animal
Id integer
AnimalCategoryId integer -- Refers back AnimalCategory
Let's say I want to create a query with Entity Framework that would load all Animals for a specific region. The easiest thing to do is to create 3 Entities Region, AnimalCategory, and Animal and use LINQ to load the data.
But let's say I am not interested in loading any AnimalCategory information and define an Entity class just to represent AnimalCategory so that I can do the JOIN. How can I do this with Entity Framework? Even with many of its Mapping functions I still don't think this is possible.
In non Entity Framework solutions this is easy to accomplish by using INNER JOINs in SPs or inline SQL. So what are my options in Entity Framework? Shall I pollute my data model with these useless tables just so I can do a JOIN?
It's a matter of choice I guess. EF choose to support many-to-many associations with transparent junction tables, i.e. where junction tables only have two foreign keys to the associated entities. They simply didn't choose to support this far less common "skipping one-to-many-to-many" scenario in a similar manner.
And I can imagine why.
To start with, in a many-to-many association, the junction table is nothing but that: a junction, an association. However, in a chain of one-to-many (or many-to-one) associations it would be exceptional for any of the involved tables to be just an association. In your example...
Animal → AnimalCategory → Region
...AnimalCategory would only have a primary key (Id) and a foreign key (RegionId). That would be useless though: Animal might just as well have a RegionId itself. There's no reason to support a data model that doesn't make sense.
What you're after though, is a model in which the table in the middle does carry information (AnimalCategory.Name), but where you'd like to map it as a transparent junction table, because a particular class model doesn't need this information.
Your focus seems to be on reading data. But EF has to support all CRUD actions. The problem here would be: how to deal with inserts? Suppose Name is a required field. There would be no way to supply its value.
Another problem would be that a statement like...
region.Animals.Add(animal);
...could mean two things:
add an Animal and a new AnimalCategory, the latter referring to the Region.
Add an Animal referring to an existing AnimalCategory - without being able to choose which one.
EF wouldn't want to choose for some default behavior. You'd have to make the choice yourself, so you can't do without access to AnimalCategory.
Is there a way to do conditional mapping in EF 6 with Code First.
We have a table called Entity which holds Suppliers, Customers and Employees. There is a flag field which identifies if a record is supplier, customer, employee or a combination of them. So we are using enum flags. In our code we have three domain objects (Supplier, Customer and Employee). I want to map these objects to the Entity table based on the flag field. Can somebody please tell me if this is possible in EF6. Please note that we are using Code First
I'm fairly new to EF and STE's, but I've stumbled on a painful point recently, and I'm wondering how others are dealing with it...
For example, suppose I have two STE's: Employee and Project. It's a many-to-many relationship. Each entity has a navigation property to the other (i.e. Employee.Projects and Project.Employees).
In my UI, a user can create/edit an Employee and associate it with multiple Projects. When the user is ready to commit, a list of Employees is passed to the server to save. However, if an Employee is not added to the "save list" (i.e. it was discarded), but an association was made to one or more Projects, the ApplyChanges extension method is able to "resurrect" the Employee object because it was "connected" to the object graph via the association to a Project.
My "save" code looks something like this:
public void UpdateEmployees(IEnumerable<Entities.Employee> employees)
{
using (var context = new EmployeeModelContainer(_connectionString))
{
foreach (var employee in employees)
{
context.Employees.ApplyChanges(employee);
}
context.SaveChanges();
}
}
I've been able to avoid this issue to now on other object graphs by using FKs to manipulate associations as described here: http://blogs.msdn.com/b/diego/archive/2010/10/06/self-tracking-entities-applychanges-and-duplicate-entities.aspx
How does one handle this when a many-to-many association and navigation properties are involved?
Thanks.
While this answer's a year late, perhaps it will be of some help to you (or at least someone else)
The simple answer is this: do not allow Entity Framework to infer m:m relationships. Unfortunately, I'm not aware of a way of preventing this, only how to deal with it after the fact.
By default, if I have a schema like this:
Employee EmployeeProject Project
----------- --------------- ----------
EmployeeId ---> EmployeeId |--> ProjectId
Name ProjectId ----- Name
... ...
Entity Framework will see that my EmployeeProject table is a simple association table with no additional information (for example, I might add a Date field to indicate when they joined a project). In such cases, it maps the relationship over an association rather than an entity. This makes for pretty code, as it helps to mitigate the oft-referenced impedence mismatch between a RDBMS and object-oriented development. After all, if I were just modeling these as objects, I'd code it the same way, right?
As you've seen, however, this can cause problems (even without using STE's, which cause even MORE problems with m:m relationships). So, what's a dev to do?
(The following assumes a DATABASE FIRST approach. Anything else and you're on your own)
You have two choices:
Add another column to your association table so that EF thinks it has more meaning and can't map it to an association. This is, of course, bad design, as you presumably don't need that column (otherwise you'd already have it) and you're only adding it because of the particular peculiarities of the ORM you've chosen. So don't.
After your context has been generated, map the association table yourself to an entity that you create by hand. To do that, follow the following steps:
Select the association in the designer and delete it. The designer will inform you that the table in question is no longer mapped and will ask you if you want to remove it from the model. Answer NO
Create a new entity (don't have it create a key property) and map it to your association table in the Mapping Details window
Right-click on your new entity and add an association
Correct the entity and multiplicity values (left side should have your association entity with a multiplicity of *, right should have the other entity with a multiplicity of 1)
Check the option that says "Add foreign key properties to the Entity"
Repeat for the other entity in the association
Fix the property names on the association entity (if desired...not strictly necessary but they're almost certainly wrong) and map them to the appropriate columns in the Mapping Details window
Select all of the scalar properties on your association entity and set them as EntityKey=True in the Properties window
Done!
I want to use a Money value object in my application. I have found several examples of a Money datatype. But I can't figure out how to use them with EF4. I would like to store each amount as a Decimal/CurrencyCode pair (where currencycode is a string - "USD", "SEK", etc) in the database. I tried creating a complexType but I couldn't get that to work. Is this possible?
It should be definitely possible. Your complex type is just pair of decimal and string property. It is exactly what complex type are used for. Depending on your approach you must do:
Database first:
You will define your database first. Your table will contain money and varchar columns representing your new type. When you update your EDMX model from database it will include it as scalar properties to your entity. You must remove those properties. Then go to model browser and create new complex type. Return back to entity and add complex property of your new complex type. And at the end you must go to entity mapping and map your complex type to those database columns.
Here is basic tutorial from MSDN but from unknown reason they didn't include such elementary details like screenshots. Here is some video from channel9.
Model first:
This is similar to database first but you don't have to deal with database creation and mapping. It will be generated for you.
Code first (EF 4.1):
You must create separate class for your complex type and use it as property in your entity. You should not need to map it by default - mapping should be infered. If it doesn't work you can map complext type either by using ComplextTypeAttribute annotation or by defining mapping in DbModelBuilder.
I can further extend approach you need to use if you provide more details.
I am just starting to use the Entity Framework 4 for the first time ever. So far I am liking it but I am a bit confused on how to correctly do inheritance.
I am doing a model-first approach, and I have my Person entity with two subtype entities, Employee and Client. EF is correctly using the table per type approach, however I can't seem to figure out how to determine what type of a Person a specific object is.
For example, if I do something like
var people = from p in entities.Person select p;
return people.ToList<Person>();
In my list that I form from this, all I care about is the Id field so i don't want to actually query all the subtype tables (this is a webpage list with links, so all I need is the name and the Id, all in the Persons table).
However, I want to form different lists using this one query, one for each type of person (so one list for Clients and another for Employees).
The issue is if I have a Person entity, I can't see any way to determine if that entity is a Client or an Employee without querying the Client or Employee tables directly. How can I easily determine the subtype of an entity without performing a bunch of additional database queries?
Use .OfType<Client>() in your query to get just the clients. See OfType.
e.g. entities.Person.OfType<Client>() ...
Use is to test if a Person object is a specific sub-class, e.g. if (p is Employee) ...
BTW why isn't it entities.People? Did you not select the pluralization option?