Consider that i have two entities with following relationship:
Entity A <-->> Entity B (one-to-many and inverse)
Now consider that i have another entity Entity C that contains all the attributes of Entity B and some others, with following relationship:
Entity A <-->> Entity C (one-to-many and inverse)
Now i can improve the architecture by making Entity B the parent of Entity C.
Entity B
^
|
Entity C
Now, my question is, will the attribute(s) AS WELL AS the relationship(s) be inherited by Entity C? Meaning, do i still need to keep the following relationship (separately)?:
Entity A <-->> Entity C
Also, i couldn't find a good example for entity inheritance in Apple documentation for Core Data. Does anyone know of an online resource that explains this, with example (preferably)?
Yes, attributes and relationships and everything else will be inherited. Be careful though, child entities like that will share the same table in sqlite with the parent entity. So if you have C inheriting from B, then a table will be created in sqlite that has the properties for both B and C which the obvious voids in the table. This is not too much of an issue with a simple inheritance like this but if you decide to get "creative" you can end up with your entire model in one table.
Now, my question is, will the
attribute(s) AS WELL AS the
relationship(s) be inherited by Entity
C? Meaning, do i still need to keep
the following relationship
(separately)?:
Yes all attributes and relationships are inherited.
Only little documentation is available at ADC
Related
I am new to CoreData environment and I'm trying to understand how it works.
In my project, I have a superclass VetExam whose subclasses are Examination, Treatments and Vaccination, which share the same attributes of their superclass and has a reference to Pet class. On the other hand, Pet class holds an array of reference of every class except of VetExam, which should only be used for Polymorphism (so that I can use VetExam object and create a single view for each type).
Based on this model, I've tried to create entities in CoreData, but it seems that I have to specify for each type the inverse relationship for each entity. This represent a problem since from VetExam entity side the relationship is of type Pet but on Pet side is To-Many for each type of Examination, which does not allow me to get the inverse reference of VetExam.
Since this explaination can easily be misunderstood, I will show you the visual representation of it.
The problem is in VetExam entity, whose Inverse attribute is not known.
Does anyone know how to deal with this type of situation?
A preliminary note on inheritance...
Class inheritance
AND
Entity inheritance
For the second, I highlight the note in the Apple Documentation:
Be careful with entity inheritance when working with SQLite persistent
stores. All entities that inherit from another entity exist within the
same table in SQLite. This factor in the design of the SQLite
persistent store can create a performance issue.
What this means is that Core Data framework creates one large table in the SQLite database that includes the parent entity and the child entities. Such a large table inherently contains inefficiencies. While this may seem convenient for you to manage now in your model editor and in your NSManagedObject subclasses, this may cause inefficiencies / performance issues in the long run if you expect your app to persist and retrieve large amounts of data in the four entities you mention.
Advice from others is very relevant here because four separate entities will in my humble opinion be easier to manage, rather than one parent entity and three child entities. You do not have to give up the class inheritance you’ve developed in your code if you choose this option.
So, to answer your question...
My logic:
Every Pet may have many instances of VetExam during its life, but each instance of VetExam is carried out on only one Pet?
If yes, then create a one-to-many relationship between Pet and VetExam -
Pet <—>> VetExam.
Whatever occurs during the VetExam is any combination of one Examination, Treatment and/or Vaccination. That is and in an attempt to be clear, the VetExam may optionally have an examination, but it may not have a treatment or a vaccination. This is likely to change for each VetExam, therefore this is directly related to the VetExam, not the Pet.
If yes, then create optional one-to-one relationships between VetExam and the entities Examination, Treatment and Vaccination.
VetExam <—> Examination
VetExam <—> Treatment
VetExam <—> Vaccination
In this model, each entity relationship detailed above has an inverse.
Finally, it might be worth noting that in this proposed model, the relationship between a Pet and all the examinations, treatments and vaccinations it receives during its lifetime is stored against PetExam, not directly against the Pet.
I am new to the RDBMS.
I am Learning ER model in RDBMS.
In ER model, the entity is an real world object and it has an attributes.
The relationship is an mapping between entity set.
The relationship also have a the attributes.
Please explain the difference between entity and relationship.
You seem to have the definition differences available. But I assume you still do not understand the differences. Here is a very simplified example of two entities and the relationship that may exist between them:
A Bank and a Person are each an entity. The relationship that exists between a Bank and a Person is that a Person is a Customer to a Bank. Therefore Customer is the relationship. An attribute for a Person for example would be Date_of_Birth. An attribute for a Bank would be Bank_Name . An attribute for a Customer would be Customer_Bank_Acc_Number.
Update:
For those that like to pick at details here is a better relationship example:
A Person can have a relationship with a Bank of either being a Debtor or Creditor.
Update
There is also what is called an Associative Entity. Click the link for details on how that is different to an Associative Relationship.
I hope this makes sense. Cheers
I'm a bit confused about this naming convention.
What is the difference between them and are target/source interchangeable with owning/not owning?
One thing in particular is hard to understand:
"The main difference between a OneToOne and a ManyToOne relationship in JPA is that a ManyToOne always contains a foreign key from the source object's table to the target object's table, where as a OneToOne relationship the foreign key may either be in the source object's table or the target object's table"
JPA wikibooks
I can't imagine such situation in uni one-to-one
Differences between them are a little confusing. You should practice a lot to understand very well.
At first, you should understand some terminology:
Role : In every relationship there are two entities that are related to one another, and each entity is said to play a role in the relationship.
Direction : Relationships can be unidirectional or bidirectional. For e.g.. a Person has an address is normally unidirectional whereas Employee working on a project is normally bidirectional. We will look at how to identify and define directionality while coming up with a Data Model.
In order to have relationships at all, there has to be a way to create, remove, and maintain them. The basic way this is done is by an entity having a relationship attribute that refers to its related entity in a way that identifies it as playing the other role of the relationship. It is often the case that the other entity, in turn, has an attribute that points back to the original entity. When each entity points to the other, the relationship is bidirectional. If only one entity has a pointer to the other, the relationship is said to be unidirectional. A relationship from an Employee to the Project that they work on would be bidirectional. The Employee should know its Project, and the Project should point to the Employee working on it. A UML model of this relationship is shown here. The arrows going in both directions indicate the bidirectionality of the relationship (Form this book >> Pro JPA 2)
Then dive into this link (archived from the original)
I'd like to comment only the links, but I need 50 reputation
I am looking for some clarification on when to use the aforementioned relationships.
As an example i have a class A which is referenced by instances of class B.
Should this relationship be modelled OneToMany and have a collection of class B defined in class A or be modelled by ManyToOne and have a reference to class A in class B?
My question is whether the choice of relationship is entirely up to the programmer, or application dependent or JPA dependent?
OneToMany and ManyToOne are two sides of the same coin. If the association is unidirectional (that is, it's not bidirectional) then the choice of relationship is the same as choosing which side owns the relationship. Examples:
You want a unidirectional relationship from 1 A to many Bs, where A is the owner. The only choice is OneToMany.
You want a unidirectional relationship from 1 A to many Bs, where B is the owner. The only choice is ManyToOne.
If the association is bidirectional then, by assumption, you're using both OneToMany and ManyToOne. Choosing the owner side does not force you to make an either-or decision.
More reading # The Java EE 6 Tutorial: Entities.
I have an Eentity Framework model with a Table Per Hierarchy (Brand) and also a Table Per Type (Vehicle) inheritance like this:
(Vehicle and Brand are abstract classes).
So far so good, I can access derived entities on linq queries using Vehicle.OfType<> or Brand.OfType<> method.
Now, Brand entity is one to many related with Vehicle on my conceptual model, So the question is, how should I make relationships on EF model so I can keep using navigation properties between Vehicle and Brand but at the same time keep the consistency of the TPH inheritance on Brand?, my first approach was to relate only derived clases, like:
But if I do this, I have no access to Brand directly from Vehicle, so I would have to do a double relation (between derived and base), like:
This works for me now, but I still have a duplicated relationship somehow, is this right?, do you have a better approach?, am I'm making some silly mistake on my modelling?
It seems to me that the reason you are running into cross-linking in your model is because you are artificially separating Brand and Vehicle as top-level sibling entities. If you start with Brand, which seems essentially equivalent to Make, that becomes the true top-level entity. There is no need to separate Make for each vehicle type (car, motorcycle, truck, etc.); just introduce the entity Model between Make and Vehicle and I think that solves most of your cross-linking problems.
Then the relationships aren't strictly parent-child, but are more accurate as composition. So you have Make, which has a one-to-many composite relationship to Model, which in turn has a one-to-many composite relationship to Vehicle. Vehicles are instances of a Model, so there isn't really a parent-child relationship there either. With this structure, there is no need to branch the EF for each type of Vehicle, because that is just part of what is described by the Model entity.
I hope my answer is helpful and that I haven't missed any of the essential points of what you are trying to model-