Is it possible to create a one-to-one relationship on already defined entity properties? I've got the following entities:
public class Asset
{
public int AssetId { get; set; }
public int OwningCustomerId { get; set; }
public virtual Customer OwningCustomer { get; set; }
}
public class CustomerAsset
{
public int CustomerId { get; set; } // Primary key 0
public int AssetId { get; set; } // Primary key 1
public int CustomerAssetStatusId { get; set; }
public string Name { get; set; }
public virtual Customer Customer { get; set; }
public virtual Asset Asset { get; set; }
}
CustomerAsset represents various states of ownership of an asset. Although the same AssetId can exist multiple times in this table, there will only ever be one instance where CustomerAssetStatusId = 1. In order to have quick access to the asset's current owner, I also maintain OwningCustomerId in the Asset table.
I would like to have a navigation property from Asset to CustomerAsset called OwningCustomerAsset. This is a one-to-one relationship. Unfortunately, I don't know how to define it since the foreign key fields are already defined in Asset. Typically, I would create a relationship like this on Asset:
HasRequired(e => e.OwningCustomerAsset).WithRequiredDependent().Map(m => m.MapKey("OwningCustomerId", "AssetId"));
Of course this results in the following error: "Each property name in a type must be unique. Property name 'OwningCustomerId' was already defined."
How can I tell EF that OwningCustomerId/AssetId is the foreign key to CustomerAsset?
If you can modify your schema, I suggest making asset ownership a property of the asset itself and not of the customer-asset relationship. Having a non-nullable FK field from Asset to Customer (e.g. Owner_CustomerID) enforces the "one owner only" constraint and the owner can be easily loaded from CustomerAsset.Asset.Owner (or whatever you choose to name it). This schema change will greatly simplify your queries.
Furthermore, this will allow you to add a navigation property to Customer that references all owned assets; Customer.OwnedAssets, for example
Update:
Also add navigation property Asset.OwningCustomerAsset (or whatever you wish) from Asset to CustomerAsset using the new Owner_CustomerID field by way of compound FK ( AssetId, Owner_CustomerId ) and resulting navigation property
Since I came down this road needing a navigation property on the other end (which I don't need anymore), I was stuck with the one-to-one mindset. The following gets me what I want:
HasRequired(e => e.OwningCustomerAsset).WithMany().HasForeignKey(e => new { e.OwningCustomerId, e.AssetId });
Related
I am using Entity Framework 6 Code First. I have two Entities:
public class User
{
[Key]
public Guid Id { get; set; }
public string Name { get; set; }
public string Mail { get; set; }
public DateTime PwLastSet { get; set; }
public virtual ICollection<Group> Groups { get; set; }
}
and
public class Group
{
[Key]
public Guid Id { get; set; }
public string Name { get; set; }
public string Description{ get; set; }
public int GroupType { get; set; }
public virtual ICollection<User> Members { get; set; }
}
in a many-to-many relationship and a joining entity GroupUsers which is generated automatically by EFCore and only includes the two Key properties from the original entities which make the primary key as well as foreign keys from the two base Entities. All this is fine and the database objects including the joining table have been created in the migration with no issues.
The problem starts when trying to Insert data into either of the two tables using EFCore.
private async Task SynchronizeUsersAsync()
{
var localUsers = await _userRepo.ListAllAsync();
List<User> remoteUsers = _usersHelper.GetUsers();
var toAdd = new List<User>();
foreach (var remoteUser in remoteUsers)
{
var localUser = localUsers.FirstOrDefault(x => x.Id.Equals(remoteUser.Id));
if (localUser == null)
{
toAdd.Add(remoteUser);
}
else if (!localUser.Equals(remoteUser))
{
_mapper.Map(remoteUser, localUser);
}
}
await DbContext.Set<User>().AddRangeAsync(toAdd);
await DbContext.SaveChangesAsync();
//delete is irrelevant at the moment
}
I get an exception:
System.InvalidOperationException: The instance of entity type 'Group' cannot be tracked because another instance with the same key value for {'Id'} is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached.
As every User potentially belongs to multiple Groups this is also supposed to insert a new row into the joining table GroupUsers for every object in ICollection<Group> Groups of a User.
I have also tried replacing .AddRangeAsync() with .AddAsync() to Insert on every new User which ended up with the same exception.
The problem occurs because of _mapper.Map. When it copies the Groups collection, it also copies every Group inside of it instead of using the same reference. This way you add different group objects to the DbContext, but they have the same Id.
The solution would be to create a mapper configuration for Groups field to make a shallow copy instead of a deep copy. In other words copy the reference to the collection (or create a new collection, but copy the references of the objects without copying them). You should probably do the same for Users field in the Group class.
I updated the title to be more relevant.
I have two objects and wish to use EF Core to map them to a database using code first.
I have shortened them a bit for brevity.
WeddingDetails
{
VenueDetails ReceptionDetails { get; set; }
VenueDeails WeddingDetails { get; set; }
}
VenueDetails
{
string StreetAddress { get; set; }
string PostCode { get; set; }
public int WeddingDetailsId { get; private set; }
public WeddingDetails WeddingDetails { get; private set; }
}
However when attempting to do the initial migration I am getting the following error from Entity Framework Core:
Unable to determine the relationship represented by navigation 'WeddingDetails.ReceptionDetails' of type 'VenueDetails'. Either manually configure the relationship, or ignore this property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.
The issue is that both receptiondetails and weddingdetails are the same object and this is causing a conflict when Entity Framework Core tries to convert this to a migration, I believe.
Could anyone please help me understand what configuration options I must add to the OnModelCreating method using Fluent API/changes I need to make to the objects to smooth out this conflict, thanks.
Edit
I actually resolved this by creating a parent class that both reception and wedding derived from, entity framework was then able to differentiate the two objects and kept them in the table using a discriminator.
So I updated it like below
VenueDetails
{
string StreetAddress { get; set; }
string PostCode { get; set; }
}
WeddingVenueDetails : VenueDetails
{
}
ReceptionVenueDetails : VenueDetails
{
}
EF was then able to parse this. The reason I have given the answer to Steve, instead of answering this myself, is that this was not the approach I have decided to go with, but more what he suggested, of actually breaking both of the classes into their own objects to allow for future flexibility and decoupling
The relationships here are a little bit backwards in terms of where the FK in the relationship will be located. One-to-One relationships are normally associated by PKs on both sides, however EF can be configured to establish a one -to-one relationship using either a one-to-many db-side relationship (FK on Right-side table) or a many-to-one db-side relationships (FK on Left-side table)
For example, given your intended structure /w Wedding and Venues, you want a single Wedding to point to a WeddingVenue and a ReceptionVenue. The next question would be where the FKs should reside. In the case of the WeddingVenue you have set this up as a one-to-many FK with the WeddingID in the Venue table. However, this leaves the ball hanging for how the database is going to figure out the FK for the ReceptionVenue.
You can solve this by swapping to a many-to-one FK where the WeddingVenueId and ReceptionVenueId are part of the Wedding table/entity. If we expose the FK properties as an example in the entity:
public class WeddingDetails
{
[Key]
public int WeddingDetailsId { get; set; }
// ... wedding fields...
public int WeddingVenueId { get; set; }
public int ReceptionVenueId { get; set; }
[ForeignKey("WeddingVenueId")]
public virtual VenueDetails WeddingVenueDetails { get; set; }
[ForeignKey("ReceptionVenueId")]
public virtual VenueDetails ReceptionVenueDetails { get; set; }
}
Now this technically sets up two many-to-one relationships where the model configuration would look something like:
modelBuilder.Entity<WeddingDetails>()
.HasOne(x => x.WeddingVenueDetails)
.WithMany();
However, we want a WeddingDetails instance on VenueDetails. There is another consideration in that ReceptionVenueDetails is a VenueDetails, that venue will not have a WeddingDetails reference, so it probably makes sense to leave this relationship as a many-to-one.
The issue here is that there is nothing stopping the same venue record from:
Being used for both the Wedding venue and Reception venue.
Being used as the Wedding and/or Reception venue for a completely different wedding.
The implications of this is that if two weddings get linked to the same venue record, updates on one wedding's venue would be reflected on any other weddings linked to that same record. Even if we were to configure EF to expect these to be a one-to-one relationship (which we can using EF Core 5/6 using HasPrincipalKey) the fact is that the database FK constraints will always allow two weddings to reference the same Venue record. This forms a Faux one-to-one relationship.
The way to enforce this as a proper one-to-one would be to either have the venue details (for both wedding and reception) embedded in the Wedding table (as an same-table owned type in EF Core) or, establish a WeddingVenueDetails as a one-to-one relationship table. (Either using a standard one-to-one PK relationship or separate table owned type)
What this would look like from entities:
public class WeddingDetails
{
[Key]
public int WeddingDetailsId { get; set; }
// ... wedding fields...
public virtual WeddingVenueDetails WeddingVenueDetails { get; set; }
}
public class WeddingVenueDetails
{
[Key]
public int WeddingDetailsId { get; set; }
string WeddingStreetAddress { get; set; }
string WeddingPostCode { get; set; }
string ReceptionStreetAddress { get; set; }
string ReceptionPostCode { get; set; }
public virtual WeddingDetails WeddingDetails { get; set; }
}
Here WeddingVenueDetails is responsible for both the wedding and reception venue details for a single, specific wedding. We can set up a proper one-to-one relationship between the two:
modelBuilder.Entity<WeddingDetails>()
.HasOne(x => x.WeddingVenueDetails) // or OwnsOne()
.WithOne(x => x.WeddingDetails);
In the database table, the WeddingDetailsId serves as the unique PK between both tables and the FK constraint. This enforces a pure one-to-one relationship where any Wedding's venue details can be safely edited independently of possibly affecting any other wedding's venue.
Similarly if you want to separate the WeddingVenue and ReceptionVenue, you can do this, however each would need its own separate table with a WeddingDetailsID as the PK and set up with the same HasOne/WithOne. (WeddingVenueDetails and ReceptionVenueDetails tables) They cannot be the same VenueDetails table/entity to enforce the pure one-to-one relationship with a WeddingDetails. (There is no such thing as a One-to-Two or One-to-Zero-or-Two relationship.) This may seem non-ideal since these two tables would effectively share the same columns, but from a storage POV it's essentially the same whether 2 records get stored in 1 table, or 1 record gets stored in each of two tables. It also accommodates future flexibility where you may have venue fields that apply only to Weddings or Receptions which can be added to their appropriate table without resorting to conditional Null-able columns. Worst case you end up having to resort to null-able columns that are implied to be required by one or the other with nothing to actually enforce that in the database. Separate tables are also better if something like the Reception is optional to avoid rows with a bunch of empty columns if in a single Venues table.
I am working on a basic group chat system, for which I created these classes:
public class Role
{
public Guid Id { get; set; };
public string Username { get; set; }
}
public class Message
{
public Guid Id { get; set; };
public Role Author { get; set; }
public Conversation Conversation { get; set; }
public DateTime Date { get; set; }
public string Text { get; set; }
}
public class Conversation
{
public Guid Id { get; set; };
public IList<ConversationParticipant> ConversationParticipants { get; set; };
public IList<Message> Messages { get; set; };
}
public class ConversationParticipant
{
public Conversation Conversation { get; set; }
public Role Role { get; set; }
}
We are using EF Core 3.1 Code-First with migrations.
I am looking for a way to make Message.Author a required property, which should lead to a column in table Message that is created as AuthorId NOT NULL.
I tried:
public static void Map(this EntityTypeBuilder<Message> builder)
{
builder.HasOne(m => m.Author);
}
As this is applied using Add-Migration and Update-Database, the database column AuthorId is created, but with NULLs allowed.
There does not seem to be a method IsRequired() that I can add after HasOne().
I also tried:
public static void Map(this EntityTypeBuilder<Message> builder)
{
builder.Property(m => m.Author).IsRequired();
}
but that fails saying
The property 'Message.Author' is of type 'Role' which is not supported by current database provider. Either change the property CLR type or ignore the property using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating'.
Doing .HasOne(...) followed by .Property(...).IsRequired() also does not work:
'Author' cannot be used as a property on entity type 'Message' because it is configured as a navigation.
I managed to make Message.Conversation required through this:
public static void Map(this EntityTypeBuilder<Conversation> builder)
{
builder.HasMany(c => c.Messages) // A conversation can have many messages
.WithOne(e => e.Conversation) // Each message belongs to at most 1 conversation
.IsRequired(); // A message always has a conversation
}
However I'd rather not make Role aware of Messages, as I will never want to retrieve Messages directly from a Role (this will happen through Conversations and Participants).
My ultimate question is: Is there a way to make Message.Author required (NOT NULL), without linking Message and Role together in a full 1-to-many relationship with a Messages property in Role?
What about adding Role's foreign key to Message and then requiring that property to not be null? Something like:
// MessageConfiguration.cs
builder.Property(b => b.RoleId).IsRequired()
While the answer by #Ben Sampica was helpful and got me where I needed to be, the comments by #Ivan Stoev provided details and clarity that made me think that a more comprehensive answer would be useful.
There are multiple ways to make a foreign key column required (NOT NULL) in the generated table.
The simplest is to put [Required] on the navigation property:
public class Message
{
// ...
[Required] public Role Author { get; set; }
// ...
}
This will cause EF to create a shadow property AuthorId of type Guid because Message.Author is a Role and Role.Id is of type Guid. This leads to UNIQUEIDENTIFIER NOT NULL in case of SQL Server.
If you omit [Required] then EF will use Guid?, which leads to UNIQUEIDENTIFIER NULL, unless you apply one of the other options.
You can use an explicit Id property with a type that can't be null:
public class Message
{
// ...
public Guid AuthorId { get; set; }
public Role Author { get; set; }
// ...
}
Note (i) - This only works if you follow EF Core shadow property naming rules, which in this case means you must name the Id property nameof(Author) + nameof(Role.Id) == AuthorId.
Note (ii) - This will break if one day you decide to rename Author or Role.Id but forget to rename AuthorId accordingly.
If you can't or don't want to change the Model class, then you can tell EF Core that it needs to treat the shadow property as required:
builder.Property("AuthorId").IsRequired();
The same Notes apply as listed at 2, with the addition that you could now use nameof() to reduce the effort and the risks.
In the end I decided to use the [Required] approach, because
It is simple and descriptive,
No effort needed to think of which shadow property name to use,
No risk of breaking the shadow property name later on.
This may apply sometimes, not always:
Input forms may use the Model class attribute to check if a property is required. However it may be a better approach to build your forms around DTO classes, and then an attribute on an EF Model class may provide no worth for your forms.
How would you delete a relationship assuming you had the 2 entities, but did not have the 'relationship' entity?
Assuming the following entities...
Model classes:
public class DisplayGroup
{
[Key]
public int GroupId { get; set; }
public string Description { get; set; }
public string Name { get; set; }
public ICollection<LookUpGroupItem> LookUpGroupItems { get; set; }
}
public class DisplayItem
{
[Key]
public int ItemId { get; set; }
public string Description { get; set; }
public string FileType { get; set; }
public string FileName { get; set; }
public ICollection<LookUpGroupItem> LookUpGroupItems { get; set; }
}
public class LookUpGroupItem
{
public int ItemId { get; set; }
public DisplayItem DisplayItem { get; set; }
public int GroupId { get; set; }
public DisplayGroup DisplayGroup { get; set; }
}
Here is the code for deleting a relationship. Note: I do not want to delete the entities, they just no longer share a relationship.
public void RemoveLink(DisplayGroup g, DisplayItem d)
{
_dataContext.Remove(g.LookUpGroupItems.Single(x => x.ItemId == d.ItemId));
}
The method above causes an error:
System.ArgumentNullException occurred
Message=Value cannot be null.
It looks like this is the case because LookUpGroupItems is null, but these were called from the database. I would agree that I do not want to load all entity relationship objects whenever I do a Get from the database, but then, what is the most efficient way to do this?
Additional NOTE: this question is not about an argument null exception. It explicitly states how to delete a relationship in Entity Framework Core.
The following is not the most efficient, but is the most reliable way:
public void RemoveLink(DisplayGroup g, DisplayItem d)
{
var link = _dataContext.Find<LookUpGroupItem>(g.GroupId, d.ItemId); // or (d.ItemId, g.GroupId) depending of how the composite PK is defined
if (link != null)
_dataContext.Remove(link);
}
It's simple and straightforward. Find method is used to locate the entity in the local cache or load it the from the database. If found, the Remove method is used to mark it for deletion (which will be applied when you call SaveChanges).
It's not the most efficient because of the database roundtrip when the entity is not contained in the local cache.
The most efficient is to use "stub" entity (with only FK properties populated):
var link = new LookUpGroupItem { GroupId = g.GroupId, ItemId = d.ItemId };
_dataContext.Remove(link);
This will only issue DELETE SQL command when ApplyChanges is called. However it has the following drawbacks:
(1) If _dataContext already contains (is tracking) a LookUpGroupItem entity with the same PK, the Remove call will throw InvalidOperationException saying something like "The instance of entity type 'LookUpGroupItem' cannot be tracked because another instance with the key value 'GroupId:1, ItemId:1' is already being tracked. When attaching existing entities, ensure that only one entity instance with a given key value is attached."
(2) If database table does not contain a record with the specified composite PK, the SaveChanges will throw DbUpdateConcurrencyException saying "Database operation expected to affect 1 row(s) but actually affected 0 row(s). Data may have been modified or deleted since entities were loaded. See http://go.microsoft.com/fwlink/?LinkId=527962 for information on understanding and handling optimistic concurrency exceptions." (this behavior is actually considered a bug by many people including me, but this is how it is).
Shorty, you can use the optimized method only if you use short lived newly create DbContext just for that operation and you are absolutely sure the record with such PK exists in the database. In all other cases (and in general) you should use the first method.
the code from the sample:
public class Student
{
public Student() { }
public int StudentId { get; set; }
public string StudentName { get; set; }
public int StdandardRefId { get; set; }
[ForeignKey("StandardRefId")]
public virtual Standard Standard { get; set; }
}
public class Standard
{
public Standard()
{
StudentsList = new List<Student>();
}
public int StandardId { get; set; }
public string Description { get; set; }
public virtual ICollection<Student> Students { get; set; }
}
Imagine an scenario where you want to set the relationship but you don't have the Standard instance in memory, the only you have is the StandardId. This would require you to first execute a query on the database to retrieve that Standard instance you need, so that you can set the Standard navigation property. There are times when you may not have the object in memory, but you do have access to that object’s key value. With a foreign key property, you can simply use the key value without depending on having that instance in memory:
int standardId=2;
//...
student.StdandardRefId =standardId;
In summary, whit the FK property you'll have two options to set the relationship.
My recommendation is when you need to create a new Student related to an existing Standard, set the FK property and not the navigation property. There are scenarios where Entity Framework will set the Standard’s state to Added even though it already exists in the database, which can cause duplicates in the database. If you are only working with the foreign key, you can avoid this problem.
By specifying the virtual Standard Standard property, you have told Entity Framework that your Student object is related to zero or one Standard objects.
In order for Entity Framework to automatically retrieve this object using Lazy Loading (Since you specified virtual), you must also tell it exactly how the objects relate by giving it a foreign key annotation. Entity Framework will automatically attempt to find a matching Standard object in the context whose primary key matches your foreign key when you try to access the Standard property for the first time.
If you do not specify a relationship, you can manually retrieve the Standard object and use the set accessor of your property.