In my EDM I have two entities Contact and Address. In my Address entity I have contactID foreign key. How can I add constraint that prevents me from deleting a contact if that contact is still being used in some Address entity?
Thanks
If you are defining your database and generating the EF model from there then add a constraint to your database using syntax like:
ALTER TABLE [Address] WITH CHECK ADD CONSTRAINT [Contact] FOREIGN KEY([ContactID]) REFERENCES [Contact] ([ID])
Then add the tables back to your EF designer and it should recognize the association and add the necessary properties required to support the constraint.
Related
In JPA project, when I generate tables for entities I can't see any table foreign key constraints created on database side.
I concluded that JPA cannot create table foreign key constraints in database and that referential integrity is enforced in JPA (on application side) and not by database.
Can someone confirm if this is so?
According to the JPA 2.2 specification, the persistence manager should create foreign key constraints. For example in the case of a one-to-one mapping:
Assuming that:
Entity A references a single instance of Entity B.
Entity B references a single instance of Entity A.
Entity A is specified as the owner of the relationship.
The following mapping defaults apply:
Entity A is mapped to a table named A.
Entity B is mapped to a table named B.
Table A contains a foreign key to table B. The foreign key column name is formed as the concatenation of the following: the name of the relationship property or field of entity A; the name of the primary key column in table B. The foreign key column has the same type as the primary key of table B and there is a unique key constraint on it.
I'm trying to understand On Delete and On Update foreign key functions in Entity Framework.
I have a database first (sqlite) model consisting of two tables:
Control
ID
Name
ControlTypeID
and
ControlType
ID
Name
The foreign key constraints on ControlTypeID are set to "On Delete Set Default" and "On Update Cascade." Default is set to "1" on ControlTypeID
The tables are bound to a datagridview. When I delete a ControlType (say, ID=2) rather than Control.ControlTypeID being set to 1, it is set to null.
Does entity framework not preserve the database foreign key constraint rules when it constructs the entity model?
Entity Framework 6 Casscading Deletes and DropForeignKey fails on auto generated constraint name
I've been running into a bit of an issue with Entity Framework and cascade deletes between two tables on several one-to-many relationships.
Initially it looked like the correct path to take was to configure the table mappings with the OnModelCreating method of DbContext turning off cascade delete in a manner such as
modelBuilder.Entity<SourceTable>()
.HasOptional(x => x.NavigationProperty)
.WithOptionalDependent()
.WillCascadeOnDelete(false);
This however did not work throwing an exception stating
Cannot delete or update a parent row: a foreign key constraint fails...
More research lead me to believe that this is because all affected entities must be loaded into the context (eager fetched) so that entity framework may set the FK references to null as part of the transaction. This is not practical for my needs based on the size of the relational graph I'd be dealing with.
My next approach was to modify the Seed method of the Configuration class and run some arbitrary SQL to drop the Foreign Key constraint and re-add it as a ON DELETE SET NULL constaint. This worked in most cases, however one of the consraints has what appears to be an auto generated unpredicatable name that is diffrent on each call of Update-Database. Given that the name can't be predicted the ALTER statments aren't particualr helpful
context.Database.ExecuteSqlCommand(#"ALTER TABLE SourceTable DROP FOREIGN KEY FK_9405957d032142c3a1227821a9ed1fdf;
ALTER TABLE SourceTable
ADD CONSTRAINT FK_ReasonableName
FOREIGN KEY (NavigationProperty_Id) REFERENCES NavigationProperty (Id) ON DELETE SET NULL;");
Finally, I've taken the apprach to use the migration functionality (DbMigration) and override Up method and leveraging the DropForeignKey method along side more explicit SQL to re-add the constraint (EF does not appear to provide a factility to create a ON DELETE SET NULL constraint).
DropForeignKey("SourceTable", "NavigationProperty_Id", "DestinationTable");
Sql("ALTER TABLE SourceTable ADD CONSTRAINT FK_ReasonableName FOREIGN KEY (NavigationProperty_Id) REFERENCES DestinationTable (Id) ON DELETE SET NULL;");
This works great, up until I encounter the constraint with the auto generate name. At this point the DropForeignKey method fails with an exception that is swallowed up by
System.Runtime.Serialization.SerializationException: Type is not resolved for member 'MySql.Data.MySqlClient.MySqlException,MySql.Data...
When dumping the migration to a SQL script file it becomes clear that the DropForeignKey simply generates a FK with a more predictable, non-ambiguous byte stream array.
Is there a proper EF Code First approach to solve the problem of setting FK column values to null when deleting the refrenced row, or am I stuck having to hand code SQL in order to gain this functionality?
how to implement Foreign key relation in JPA side ( There is no foreign key for this relation in db, Db owned by another application , i cant able to change db structure( SW vendor not allowing me to do it)
Is there just no foreign key constraint, or nothing referencing the id at all?
If there is just no constraint, then it does not matter, JPA does not care if there is a constraint or not, just use the column that references the id.
If there is nothing referencing the id, then you cannot have a relationship with nothing store it. If you cannot alter the table, then perhaps you can add a new table that defines the join between the two tables (similar to a many to many, but JPA also allows a join table to be used for a one to one).
I have a table called farmers. Each farmer has a country specified that is mandatory.
When I add a new farmer to the database using antity framework, I get a violation on the country table. It looks like the entity framework wants to add the country to the country table, but I only want the guid in my farmer table:
Violation of PRIMARY KEY constraint 'PK_Country'. Cannot insert
duplicate key in object 'dbo.Country'. The statement has been
terminated.
Can somebody advise me on what I'm doing wrong? here the code for the insert:
newFarmer.Guid = Guid.NewGuid();
ents.Farmer.AddObject(newFarmer);
ents.SaveChanges();
return newFarmer;
I even checked the state of the country and it says unchanged.
One possible solution is that Entity Framework doesn't understand that your entity primary key is also the identity and should be auto-incremented. I had the same problem in an application using EF 4.1 with database first. To solve the problem, I had to::
Make sure my entities primary key had a name "ID" (to avoid putting a decorator [Key] above my Model class.
Make sure the property option "Identity" of your database system (SQL Server in my case) is set to "Yes".
Then, my EF4.1 was able to do the insert and update of my entities.
Hope this helps!