I have little bit confusing many to many relationship between 3 entities. And i want to know how can be my object model look like. I have three Entities, A,B,C and A<->B (M:N) and associate table between both, A and B, is linked with another associate table which make another 1:n relationship with third entity. I have never seen such relationship which make 1:n relationship with another associate table. For further information please have look on following diagram.
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If i talk about object model then i will say "INSTANCE_A" has many "INSTANCE_B" instance and vice versa but i do not know how can i summarize relationship for "INSTANCE_C".
Please also let me know whether definition of such relationship between all three entities is right ? i mean is there any problem in relationship design.
Thanks in advance
EDIT: All arrows denote (1:n or m:1) relationship
The data model is correct, but the object model for these tables can be kind of trucky. I'd do something like this:
One class for TBL_A, with a List attribute of TBL_B
One class for TBL_B, with a List attribute of TBL_A
One class for TBL_C_TBL_A_B, with and an attribute for TBL_B, TBL_A and TBL_C
Mapping that in an ORM framework can get funky.
This shall bring you into the right direction. Try to design a UML diagram, or ER should be ok too. Here is some paper with a Model and the corresponding Java-Code for this model http://www.csd.uoc.gr/~hy252/references/UML_for_Java_Programmers-Book.pdf. (Go to -> Class diagrams chapter).
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I have made this simple UML diagram.
I want to have a list of treatments inside class Customer.
My question is if I need another association to make this possible.
A class diagram describes classes and not objects. A TreatmentList is a TreatmentList, regardless if you look at it from the Menu or the Customer persepective.
So indeed, if you want a list of treatment "inside" the class Customer, you'll need just to add another association between the classes you want to connect.
Now I wonder if you did not try to design user interface using a class diagram:
Main -> Menu -> List 1 or List 2 looks terribly like a flow in the user interface, more than a set of related classes. You don't need this. And lass diagrams are not meant for this purpose. If you want to model an UI Menu, you'd model a menu class, i.e a general class that could instantiate any kind of menus, with a one-to-many association with MenuAction. Perhaps ShowTreamtmentList would be a specialization of such an action.
The TreatmentList and CustomerList only make sense if these are classes, i.e. they could be instantiated into one or several objects that each represent a different list. If, for an association, you'd have a multiplicity of * on the side of customer or treatment, you would not need to add a special list/set/bag in the middle.
You don't need to use an aggregate instead of an association. Aggregate are not very uselful and appear to be overused. Prefer a simple association.
one to many
many to one
many to many
one to one
Need some more clarifications about them. Referred that in many sites but cant able to get the perfect definitions. thanks in advance
I explain you these conceptual data modeling relations on the example.
One to many (1:n) means one entity can work with more ones.
Example: One employer can hire more employees.
Many to one (n:1) works same as the one above from the different view.
One to one (1:1) means one entity can work only with the other one.
Example: Every employee can work only with one computer
Many to many (n:m) means there is a combination of relations between entities.
Example: Each customer belongs to more employees and one employee serves for one or more customers.
Usually in database systems there will be created a new table holding these relations in case of m:n.
I'm looking for some suggestions for best practices around modeling the relationship between various entities and their documents (binaries such as PDF, TIFF etc). The entities are standard JPA/Hibernate stored in a PostgreSQL database. The documents themselves will be stored in MongoDb database.
The plan is to create a child entity to represent the document, which contains the id to the binary data to retrieve it as needed. But what would the relationship be?
If I simply created one if these document entities for each parent entity then a simple one to many relationship would work, but that seems to redundant.
I could simply put a "type" column that indicates which entity the document belongs to, and then query the document table with a named query of "id = ? and type = ?". I guess that would work, but there is something about that I'm not crazy about either - just can't put my finger on it :) Maybe that's just fine.
Another option I have looked at (although I admit I have never used it before, and would need to study it a bit more) is to use a unidirectional one to many with join table. However, I don't think this will work either since there is no guarantee that there wouldn't be duplicate parent keys. I use a single sequence for all basic relation tables primary keys, which should guarantee it, but it still doesn't sound like a good idea.
Finally, I have considered whether I create an entity and then extend it for each parent entity, but I think that would have the same flaw - the theoretical existence of non-unique parent ids.
Before I make a final decision, I'd like to see what other suggestions the community might have to offer.
Thanks in advance for your ideas.
If I simply created one if these document entities for each parent entity then a simple one to many relationship would work, but that seems to redundant.
I'm a bit confused. If you create a document for each parent, isn't that one-to-one and not one-to-many? It sounds like you want a one-to-many relationship. In which case, you would only create a single document for all parent entities that reference it.
In my hypothetical I have an annotated User model class. This User model also holds references to two sets:
A set of Pet objects (a Pet object is also an annotated model represented in the data layer)
A set of Food objects (a Pet object is also an annotated model represented in the data layer)
When I pull the User entity from the database (entityManager.find(User.class, id)) it will automatically fill all the User fields but it obviously wont fill the two sets.
Do I need to do entityManager.createQuery and just use a normal SQL join query then manually create the User object?
Thanks in advance
If you map your relations from User to Pet and Food using OneToMany you can chose whether to have the fields automatically collected or not.
See the API doc for javax.persistence OneToMany.
Depending on how you constructed the mapping (PK-FK or join tables etc), you may or may not get good performance with this. Having two OneToMany relations that are joined, means you may end up with a ridiculous amount of rows when you read up your user.
Mmm, No? That's probably not how you want to do it. I don't know why you say "it obviously won't fill the two sets." It's quite capable of filling in the sets for you, that's sort of the point behind using an ORM like hibernate in the first place. Your objects do what they look like they should in code and 'databasey' things are handled automatically as much as possible.
It is true that Hibernate will complain if you mark more than one collection as EAGER fetched, but it's not really clear you actually need either of them to be eager. Essentially once they are mapped, just accessing them causes the queries to be run to fill them in with data (assuming the Session is still open and so forth.) If you explain how you want it to work it would be easier to help with a solution.
I have a ManyToMany relationship between two classes, for instance class Customer and class Item. A customer can buy several items and an item can be bought by different customers. I need to store extra information in this relationship, for example the day when the item was bought. I wonder how is this usually modelled in JPA, cause I'm not sure how to express this in code. Do I have to create a new class to model all the attributes of the relationship and make a manyToMany relationship between the other classes or is a better way to do this?
Thanks
The recommended way is to create a new association class to store the needed attributes, and two one-to-many associations to the two parties involved.
I guess you will indeed have to create a new class for the relationship.
Like you said yourself, the correct way is to create a new class with the additional attributes.