It's not completely clear to me what's the diference between using AsNoTracking() on a entity that has a key, and using HasNoKey(). As I understand, when using AsNoTracking(), even though the changetracker does not track changes on the resulting objects, the objects are still kept in the DbContext's memory. If you try to Attach a new object with the same key as one already in memory, you get an error. (correct me if i'm wrong).
So is the behavior the same if you use HasNoKey (former DbQuery<>)?
Introducing ModelBuilder.Entity<>().HasNoKey() is one of the breaking changes in EF Core 3.0 as stated:
A query type now becomes just an entity type without a primary key. Keyless entity types have the same functionality as query types in previous versions.
Query types were a means to query data that doesn't define a primary key in a structured way. That is, a query type was used for mapping entity types without keys (more likely from a view, but possibly from a table) while a regular entity type was used when a key was available (more likely from a table, but possibly from a view).
You said that:
If you try to Attach a new object with the same key as one already in memory, you get an error. (correct me if i'm wrong). So is the behavior the same if you use HasNoKey (former DbQuery<>)?
-Tracking in EF Core transaction are based on Entity Primary key. As the above documentation clearly stating that EntityType with .HasNoKey() is QueryType which does not have any key defined. So tracking is completely void in case of EntityType with HasNoKey().
For more details : Query types are consolidated with entity types
Related
I want to use the EF Core HasData method to seed a database with reference data. Two of the models I want to seed follow the Table per Hierarchy (TPH) pattern in the sense that there is a none abstract base type and another none abstract derived type. These models in the database exist in a table named after the base type with a discriminator column. I would like to add data seeding for this table. However I’m struggling to find any guidelines of how to do so in the EF Core 6 documentation.
I have a few related questions:
Do I seed all the data using the HasData method on an EntityTypeBuilder<BaseType> or do I need to split the seeding into one HasData on that class and another on EntityTypeBuilder<DerivedType>?
I understand using TPH will add a shadow discriminator property and that, potentially, I’ll have to add that in the data seeding. Does that mean I have to use anonymous types to specify that property (doesn’t seem very elegant) and if so, can I get the autogenerated discriminator name using a method (typing it manually sounds like a risk as what if EF Core changes the discriminator name convention?)?
Should I be avoiding using TPH on reference tables altogether? Is there something else I should do instead?
After going through my options with trial and error, I have come to the following solution. It may not be perfect, but it deals with all of my concerns.
Do I seed all the data using the HasData method on an EntityTypeBuilder<BaseType> or do I need to split the seeding into one HasData on that class and another on EntityTypeBuilder<DerivedType>?
You have to do it on both classes, attempting otherwise throws on creating the migration:
The seed entity for entity type 'BaseType' cannot be added because the value provided is of a derived type 'DerivedType'. Add the derived seed entities to the corresponding entity type.
I understand using TPH will add a shadow discriminator property and that, potentially, I’ll have to add that in the data seeding. Does that mean I have to use anonymous types to specify that property (doesn’t seem very elegant) and if so, can I get the autogenerated discriminator name using a method (typing it manually sounds like a risk as what if EF Core changes the discriminator name convention?)?
I have added a property to the BaseType class and defined it as the discriminator, so now I can specify the discriminator values directly without using anonymous type and I am in control of the discriminator values. Gert Arnold points out in this other answer why this might not be appropriate. However I have used the following method described in this EF Core Github issue to hopefully mitigate the concerns he raised.
Should I be avoiding using TPH on reference tables altogether? Is there something else I should do instead?
This is still unanswered. But I am feeling a lot more confident it is a good approach now.
In Orion, is it possible to change the type of an entity?
Related question, it is possible to change the type of an attribute?
Edit: Changing the type of an entity doesn't seem to be possible. So creating a new entity seems to be a solution. But how to preserve dateCreated and dateModified?
In Orion, is it possible to change the type of an entity
Not directly. Entity id and entity type are immutable. However, you can create a copy of the entity with the new type, then delete the old entity. That will have effectively the same effect.
Related question, it is possible to change the type of an attribute?
Yes, it is. When you update an attribute, not only the value but also the type (and metadata) can be changed.
EDIT: dateModified and dateCreation attributes and metadata are designed to be managed by Context Broker automatically. Clients cannot modify them, they are "read-only". The idea is that a given context consumer client (which, in principle, is independent of the context producer client creating/updating the entity) has a trustable timestamp that nobody could alter.
So, you have basically two alternatives:
Use your custom timestamp attributes and metadata. They can be "moved" to the new entity when you create it. However, CB will not maintain them automatically so your application would have to do it.
Use an out-of-API process, moving the entity at DB level. However, this can be complicated, as you need direct access to DB.
EDIT2: for the second case (DB based process) take into account the Orion DB model. In particular:
_id.type is for the entity type
creDate is for the entity creation date
modDate is for the entity modification date
attrs.A.creDate is for the attribute A creation date
attrs.A.modDate is for the attribute A modification date
I'm using Entity Framework 5, code first approach. As there's no built in support for updating child entities in disconnected scenario, I'm building my own mechanism to do that. At some point I need to get the property of an entity with which it has a Foreign Key relationship with another (principal) entity. I've tried to get access to CSpace through
((IObjectContextAdapter)dbContext).ObjectContext.MetadataWorkspace.GetItems<MyEntity>(System.Data.Entity.Core.Metadata.Edm.DataSpace.CSpace)
but here I got a warning that said there's no implicit conversion between MyEntity and System.Data.Entity.Core.Metadata.Edm.GlobalItem.
I can't look for a property that has Foreign Key attribute because in most of my entities I use EF convention to get foreign keys automatically. So how one would go about finding which property is used for foreign key relationship.
Thanks to #octavioccl's post I was able to do what I want. So I was in the right path to look inside ObjectContext.
I want to create an entity in EF without a public key. The backing table has got a non-unique clustered key, but the data in the table conceptually doesn't have a unique primary key it can use.
It looks like EF really doesn't like this. Is there any way of getting EF to accept that the table has no primary key and make it work with it anyway, with no performance hit? I don't care if the result is read-only.
As I understand it, as the Entity Framework is based on the Domain Driven Design concept of Entities, each Entity by definition must have a unique identifier. If the concept which the data in your table represents does not conceptually have a unique identifier then it is not an Entity, in the sense intended by the framework.
With this in mind I'd define a Stored Procedure, make it available through my object context, then make the objects encapsulating this data available via a class which lazy-loads the data, manually maps it into the objects you're using and presents it in a read-only manner.
You may also be able to accomplish this by exposing a view and then mapping your entity to the view.
For part of the project I'm currently working on, I have a set of four tables for syndicatable actions. One table is the abstract base for the other three, and each table is represented in my EF model like so:
EF Model -- Actions http://chris.charabaruk.com/system/files/images/EF+Model+Actions.png
There are two problems that I'm currently facing with this, however. The first problem is that Actor (a reference to a User) and Subject (a reference to an entity of the class associated with each type of action) are null in my subclasses, despite the associated database columns holding valid keys to rows in their associated tables. While I can get the keys via ActorReference and SubjectReference this of course requires setting up a new EF context and querying it for the referenced objects (as FooReference.Value is also null).
The second problem is that the reciprocal end of the relationship between the concrete action classes and their related entity classes always turn up nothing. For example, Task.RelatedActions, which should give me all TaskAction objects where Subject refers to the particular task object on which RelatedActions is called, is entirely devoid of objects. Again, valid rows exist in the database, Entity Framework just isn't putting them in objects and handing them to me.
Anyone know what it is I'm doing wrong, and what I should do to make it work?
Update: Seems that none of the relationship properties are working in my entity model any more, at all. WTF...
I think the issue you are experiencing here is that by default the EF does not automatically load related entities. If you load an entity, the collection or reference to related entities will be empty unless you do one of the following things:
1) Use eager loading in order to retrieve your main entity and your related entity in a single query. To do this, modify your query by adding a call to the Include method. In your sample above, you might use the following query:
from a in context.Actions.Include("Actor") select a
This would retrieve each of the actions with the related Actor method.
2) Use explicit lazy loading to retrieve the related entity when you need it:
action1.ActorReference.Load()
In the version of the EF which will ship with .Net 4.0, you will also have the following additional option:
3) Turn on implicit lazy loading so that related entities will automatically be retrieved when you reference the navigation property.
Danny