i have below 3 tables
Table A: Table B: Table C:
id TableA id(foreign key) id
type TableC id(foreign key) name
i have requirement where i have to fetch everything from tableA but when i fetch i have to fetch the name associated from Table C based on the mapping from TableB.
it's a oneToOne mapping. i tried adding Embeddable , joincolumn but nothing works.
i was referring this link - https://vladmihalcea.com/the-best-way-to-map-a-many-to-many-association-with-extra-columns-when-using-jpa-and-hibernate/ and implemented same but still no luck.
how would i achieve this through spring boot JPA?
Can somebody help me on this?
Related
We have an existing database which we'd like to connect to using code-first EF.
An issue we have come across is in the following scenario...
**TABLE 1**
DebtorID Int PK (identity column)
Type Int PK FK ( value would be '1' - FK to Table 4)
**TABLE 2**
CreditorID Int PK (identity column)
Type Int PK FK (value would be '2' - FK to Table 4)
**TABLE 3**
TransactionID Int PK
ParentID Int FK
ParentType Int FK
-----------------------
**TABLE 4**
Type Int PK (identity column)
Description Varchar(max)
Table 3 has foreign key links to multiple tables (Table 1, Table 2, etc...). The key is unique by the type column. Constraints have been altered here to allow this scenario.
Q. How can we implement this schema using code-first? We have looked at Data Annotations, and also Fluent API, but can't come up with a solution other than to alter the database schema (which would be a big headache).
As it stands, we only seem to be able to link to a single foreign table.
The solution also has to account for other tables (not shown here) where the primary keys for Table 1 etc... are part of the primary key.
A real world view of our schema, and the issue with code-first.
Current schema:
http://oi60.tinypic.com/2aeug4m.jpg
Code-First schema:
http://oi62.tinypic.com/9schmc.jpg
I have found the answer using the Fluent Api - was much easier than I thought.
Simply in the OnModelCreating() method, I needed to add multiple ForeignKey entries to cover each link.
modelBuilder.Entity(Of TABLE_3).HasRequired(Function(p) p.TABLE_1).WithMany().HasForeignKey(Function(p) New With {p.ParentID, p.ParentType})
modelBuilder.Entity(Of TABLE_3).HasRequired(Function(p) p.TABLE_2).WithMany().HasForeignKey(Function(p) New With {p.ParentID, p.ParentType})
This was in addition to any configuration I did using Data Annotation with regards to setting up primary keys and navigational properties (e.g. navigation properties for TABLE_1 and TABLE_2)
I'm having a hard time finding the exact answer to this question, so my apologies if this is redundant.
So I have 3 tables defined such that:
Person :PersonId, FirstName, LastName
Company: CompanyId, CompanyName
Order: OrderId, PersonId, CompanyId
On the Order table, there is a foreign key defined on the PersonId and CompanyId columns, thus, my Order entity class generated by EF has a navigation properties of type Person (not PersonId) and Company.
So, to insert into the Order table, I first need to query the person and company tables to get the person and company entities. Then I can construct the Order object using the Person and Company entities and save it to the db.
In my scenario, I am being passed a PersonId and CompanyId.
In classic SQL I would just do INSERT INTO Order Set (CompanyId, PersonId) - 1 database call. But with EF, I have to do 3 db calls. This seems like overkill.
Is there any way around this?
PS - I'm using EF 6. I know I could generate an expression and make it single call..but that would still yield two subselects.
You can just include foreign key properties in addition to the navigation properties and then set them using the ids you have. If you do this will not have to go to the database to get related entities for just a sake of setting the relationship.
i need to map some parent children relationship on top of a legacy database, the problem is how the tables are structured.
tableA
id
name
tableB
id
tableA_parentId
tableA_childId
As showed I have two foreign key from tableA to tableB
Is there a way to map this relationship with EF i'm using the code first approach and have a configuration class per table
Suppose the following database schema:
Table A: AId (PK)
Table B: BId (PK)
Table C: CId (PK)
Table AB: AId, BId (composite PK, FKs to A and B), Data
Table BC: BId, CId (composite PK, FKs to B and C), Data
Table ABC: AId, BId, CId, Data
In the database, ABC has two FKs: one to AB on AId and BId, and one to BC on BId and CId.
Use the EF Designer and attempt to create a Model from this database.
If you have Include foreign key columns in the model checked, it works; but having FK Columns in the model isn't very nice.
If you have Include foreign key columns in the model unchecked, only one of the FKs from ABC will be successfully mapped. To see what went wrong, you have to view the .edmx xml (thanks Craig!) and you see this error:
warning 6037: Foreign key constraint 'FK_ABC_BC' has been omitted from the storage model. Column 'BId' of table 'Model.Store.ABC' is a foreign key participating in multiple relationships. A one-to-one Entity Model will not validate since data inconsistency is possible.
I've read the only other mention of this problem I can find on SO, and I don't think this is the same problem. I can't see anything wrong at a database design level. I'm going to work round this for the time being by imposing surrogate keys on AB and BC, but what I'd really like to know is:
What possible data inconsistency is EF worried about happening here, if it created a model to match the database?
And is there anything I can do to persuade it that everything's going to be OK?
My opinion is that EF is too clever in this scenario and it prevents you from using entity where you can assign only one relation and make the entity non-savable because relation to second entity will not exists.
There is also possibility that EF has some internal problem with tracking state of independent associations if more than one association is based on the same foreign key column but that is just another guess. Generally database features used to map EF features cannot be shared among multiple constructions. The only exceptions I can think about now are primary keys and in their own way discriminator columns.
I would like to mention that I don't like this type of relations in database at all.
Is there any way of getting discriminated associations to work in Entity Framework 4? That is, where we have the following tables
TableA
RelatedEntityTypeId
RelatedEntityTypeKey
TableB (1)
Id
TableC (2)
Id
TableD (3)
Id
and I want to have three associations on the entity for TableA:
TableB
TableC
TableD
which are defined by the RelatedEntityTypeId and RelatedEntityTypeKey fields...when RelatedEntityTypeId = 1, then the association is to EntityB, when RelatedEntityTypeId = 2, then the association is to EntityC, etc.
Thanks.
I don't know your purpose of doing this. I have used following approach to solve similer problem.
You can define a base type for all three tables (A,B,C). And when you want retrieve a object use a generic method for all tables (which returns a base object). And then you can check the type of the returned object to get the A,B,C object.
TableBase
Id
TableB (1):TableBase
TableC (2):TableBase
TableD (3):TableBase