While I was trying to deal with many-to-many entities I had a mistake
because it has a DefiningQuery and no element exists in the element
I found an answer on stack that I have to set primary key to the table, but I am not sure which column to select as primary key, because in all articles it is said that mapping table has to be table with just two foreign keys, adding third column (for example ID) just ruins this many-to-many relationship. I have
Projects and Clients and Project_Client table where are two columns with foreign keys one to ProjectId second on ClientId. So where i have to set primary key?
My edmx:
If I add field with Id entity framework recognizes this relationship in a different way.
Related
The entity A has an OneToMany relation with the entity B. I would like to use a composite primary key for the entity B, where one of the columns is the same as the the primary key of the entity A and the second column should be generated. How can the second column be started from 1 whenever the the other involving column within the composite primary key changes?
I use Oracle data bank and could not find any solution.
Thanks in advance!
Hi all,
Can any understand what's going on here?
The case is:
There are 2 tables, called "matricula" and "pagament" with a 1:1 relationship cardinality.
Table matricula primary key composed by 3 fields "edicio","curs" and "estudiant".
Table pagament primary key, the same as above. Furthermore, it references matricula.
As shown, trying to insert a row in pagament table is rejected because it does not exists a row in table matricula. However, asking for this row returns one result.
What am I missing?
Thanks you all
Carles
The problem is that the order of the fields in both tables is not the same, and, moreover, the restriction of the foreign key in table pagament, said that
foreign key (estudiant,curs,edicio) references matricula
without specifying the matricula fields.
It's been solved by setting this restriction as
foreign key (estudiant,curs,edicio) references matricula(estudiant,curs,edicio)
In table A I've got a composite of 3 columns as a primary key. I want to have only one of these three columns as a foreign key in table B, just to make sure that the value that I insert into table B's column exists in table A.
Currently from what I've read it looks like in Entity Framework I have to add all three columns of composite PK, which is not really what I need. The latest answer that I've found was of 2015, maybe since then something changed?
I know that I can add a manual check on each insert/update call, but I don't want to do that, maybe there is more elegant way.
I am using Spring data JPA(Hibernate).
I am trying to join my tables (Table A & Table B) but on Non-Primary Columns. Is it possible to actually do that? I am trying to use referenceColumnName, but it seems to not working, giving error :
Cannot set int to Integer.
When I am removing referenceColumnName, then it is working but obviously it is joining with Primary Key. Also in case of One-to-one Bidirectional, where should I place mappedBy & JoinColumn?
The annotation #JoinColumn indicates that this entity is the owner of the relationship (that is: the corresponding table has a column with a foreign key to the referenced table), whereas the attribute mappedBy indicates that the entity in this side is the inverse of the relationship, and the owner resides in the "other" entity.
Regarding the other question of using joining tables on Non-Primary columns, there are plenty of threads why don't you go through. for example
Does the JPA specification allow references to non-primary key columns?
Is there a way to map multiple foreign keys to the same navigation property of an entity? I have a table with a self-referencing many to many relationship defined by a relationship table.
For example, I have a Person table and I want to define the different related people. I created another table, PersonRelationship, that defines those relationships with 2 foreign keys to the Person table. It doesn't matter if the person is referenced in the first or the second foreign key just that the person is in one. I would like my Person entity to have a list of all the relationships but instead it has two lists of relationships: one in which the person is in the first foreign key and another where it is the second foreign key.
How can I map these two foreign keys so that they map to the same list in the Person entity? (I am using a database first approach)