I'm wondering if it's possible to "rebuild" relationships in Core Data.
Basically, I messed up when creating my entities, getting data from a SQL server i filled my entities as tables (one table by one table) and it seems now that the relationships are not working.
Let's take an example :
I've set up my model with my two entities (department and employee), with a relation one-to-many "myRelationWithDepartment" (one department many employees), and generated the managed object subclasses accordingly.
I got table department and table employees from my SQL server and inserted all employees in my entity employee and only then (because i have to retrieve first employees) all departments in my entity department, using Core Data, saving context, etc. Everything is fine, just the relationship is not working.
Now I'm able to make a fetch request upon a department or an employee, it works fine. But if I retrieve an employee and do this :
[[anEmployee myRelationWithDepartment] departmentName];
it's returning nil, no compilation warnings or errors, it just seems that no department is linked to an employee.
So I assume that the relations are not working.
I've included in the model the "id"s I had in my SQL tables, so I'm able to link them manually (but i have multiple entities actually).
I've gone through the Core-Data guide and found this :
[aDepartment.employees addObject:newEmployee]; // do not do this!
then KVO change notifications are not emitted and the inverse relationship is not updated correctly.
Recall that the dot simply invokes the accessor method, so for the same reasons:
[[aDepartment employees] addObject:newEmployee]; // do not do this, either!
That's why I assume relationships are badly shaped. Is there a way to rebuild the relationships afterwards (since i share some id's in the model between entities)?
I dig up more in Apple's documentation and relationship has to be filled manually :
To create the relationship "link" :
anEmployee.myRelationWithDepartment = departmentObject;
Alternatively, you can use:
[department addEmployeeObject:anEmployee];
Then when fetching objects you can access properties of related entities.
The problem is that Core-Data is "sold" as everything is doing quite by itself and in reality it's much deeper as it seems at first glance.
Related
Using Core Data, I have two entities that have many-to-many relationships. So:
Class A <<---->> Class B
Both relationships are set up as 'ordered' so I can track they're order in a UITableView. That works fine, no problem.
I am about to try and implement iCloud with this Core Data model, and find out that iCloud doesn't support ordered relationships, so I need to reimplement the ordering somehow.
I've done this with another entity that has a one-to-many relationship with no problem, I add an 'order' attribute to the entity and store it's order information there. But with a many-to-many relationship I need an unknown number of order attributes.
I can think of two solutions, neither of which seem ideal to me so maybe I'm missing something;
Option 1. I add an intermediary entity. This entity has a one-to-many relationship with both entities like so:
Class A <<--> Class C <-->> Class B
That means I can have the single order attribute in this helper entity.
Option 2. Instead of an order attribute that stores a single order number, I store a dictionary that I can store as many order numbers as I need, probably with the corresponding object (ID?) as the key and the order number as the value.
I'm not necessarily looking for any code so any thoughts or suggestions would be appreciated.
I think your option 1, employing a "join table" with an order attribute is the most feasible solution for this problem. Indeed, this has been done many times in the past. This is exactly the case for which you would use a join table in Core Data although the framework already gives you many-to-many relationships: if you want to store information about the relationship itself, which is precisely your case. Often these are timestamps, in your case it is a sequence number.
You state: "...solutions, neither of which seem ideal to me". To me, the above seems indeed "ideal". I have used this scheme repeatedly with great performance and maintainability.
The only problem (though it is the same as with a to-one relationship) is that when inserting an item out of sequence you have to update many entities to get the order right. That seems cumbersome and could potentially harm performance. In practice, however, it is quite manageable and performs rather well.
NB: As for arrays or dictionaries to be stored with the entity to keep track of ordering information: this is possible via so-called "transformable" attributes, but the overhead is daunting. These attributes have to be serialized and deserialized, and in order to retrieve one sequence number you have to get all of them. Hardly an attractive design choice.
Before we had ordered relationships for more than 10 years, everyone used a "helper" entity. So that is the thing that you should do.
Additional note 1: This is no "helper" entity. It is a entity that models a fact in your model. In my books I always had the same example:
You have a group entity with members. Every member can belong to many groups. The "helper" entity is nothing else than membership.
Additional note 2: It is hard to synchronize such an ordered relationship. This is why it is not done automatically. However, you have to do it. Since CD and synchronizing is no fun, CD and synchronizing a model with ordered relationship is less than no fun.
I have two entities:
Patient
- firstName
- lastName
- scheduledAppointments <---->> Appointment
Appointment
- date
- times
- scheduledPatient <<----> Patient
Basically I have one Patient with many appointments. How do I set the scheduledPatient in the Appointments entity? I've tried this so far:
[self.appointment setScheduledPatient:[self.patientArray objectAtIndex:indexPath.row]];
self.appointment.scheduledPatient = [self.patientArray objectAtIndex:indexPath.row];
They work when I'm editing an appointment. But it returns a SIGBRT when I'm adding a new appointment.
Your code seems to be correct.
Therefore I suppose that most likely you have not defined the inverse relation properly in the .xcdatamodel file.
For what I understand you have a one-to-many relationships. That is, one Patient may have a number of Appointments. Therefore, an appointment belongs to one patient. In order for this relation to be semantically correct, you need to let it know how they relate to each other. In order to do so you need to specify what is the inverse element of each of the elements in the relation.
In the picture below you can see how a Region may have a number of states, and a state belongs uniquely to a region. Notice the arrow connecting the elements of the relation, how the "many" has a double arrow and the "one" has a single arrow.
I believe that you most likely forgot to specify this in the xcdatamodel file.
Check out this link for more information:https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreData/Articles/cdRelationships.html
Inverse Relationships
Most relationships are inherently bi-directional. If a Department has a to-many relationship to the Employees that work in a Department, there is an inverse relationship from an Employee to the Department. The major exception is a fetched property, which represents a weak one-way relationship—there is no relationship from the destination to the source (see “Fetched Properties”).
You should typically model relationships in both directions, and specify the inverse relationships appropriately. Core Data uses this information to ensure the consistency of the object graph if a change is made (see “Manipulating Relationships and Object Graph Integrity”). For a discussion of some of the reasons why you might not want to model a relationship in both directions, and some of the problems that might arise if you don’t, see “Unidirectional Relationships.”
I'm currently writing my first project using core data, and am having trouble working out how to query the relationship between some of my data.
In sql language, i have a Country table, which joins to a CountryLink M-M table containing the following fields:
countryId1
countryId2
bearing
What would be the correct way to model this in Core Data?
So far i have set up a single Country entity and a CountryLink entity (containing only a bearing field) and have added two 1-to-Many relationships from Country to CountryLink ('CountryLink1' and 'CountryLink2').
I've run the project and looked at the Sqlite db structure produced by Core Data (found here, using this sqlite gui), and the M-M join table seems correct (it contains the bearing, CountryLink1 and CountryLink2 fields), but i'm not sure how i would go about carrying out a fetch request for a single Country NSManagedObject to return an array of related Countries and their bearings?
Any help or related links would be much appreciated.
Thanks, Ted
First a word of warning:
Core Data is not SQL. Entities are not tables. Objects are not rows. Columns are not attributes. Core Data is an object graph management system that may or may not persist the object graph and may or may not use SQL far behind the scenes to do so. Trying to think of Core Data in SQL terms will cause you to completely misunderstand Core Data and result in much grief and wasted time.
See the Tequilla advice
Now, forgetting SQL and thinking in object graphs, your entities would look something like this:
Country{
someAttribute:string // or whatever
countryLinks<-->>CountryLink.country
}
CountryLink{
countryID1:string // or whatever
countryID2:string // or whatever
country<<-->Country.countryLinks
}
As you add Country and CountryLink objects you add them to the relationships as needed. Then to find CountryLink objects related to a specific Country object, you would perform a fetch on the Country entity for Country objects matching some criteria. Once you have that object, you simply ask it for the CountryLink objects in its countryLinks relationship. And your done.
The important thing to remember here is that entities in combination with managedObjects are intended to model real-world objects, conditions or events and the relationship between the same. e.g. a person and his cars. SQL doesn't really model or simulate, it just stores.
I'm reading Pro JPA 2. The book talks begins by talking about ORM in the first few pages.
It talks about mapping a single Java class named Employee with the following instance variables - id,name,startDate, salary.
It then goes on to the issue of how this class can be represented in a relational database and suggests the following scheme.
table A: emp
id - primary key
startDate
table B: emp_sal
id - primary key in this table, which is also a foreign key referencing the 'id' column in table A.
It thus seems to suggest that persisting an Employee instance to the database would require operations on two(multiple) tables.
Should the Employee class have an instance variable 'salary' in the first place?
I think it should possibly belong to a separate class (Class salary maybe?) representing salary and thus the example doesn't seem very intuitive.
What am I missing here?
First, the author explains that there are multiples ways to represent a class in a database: sometimes the mapping of a class to a table is straightforward, sometimes you don't have a direct correspondence between attributes and columns, sometimes a single class is represented by multiples tables:
In scenario (C), the EMP table has
been split so that the salary
information is stored in a separate
EMP_SAL table. This allows the
database administrator to restrict
SELECT access on salary information to
those users who genuinely require it.
With such a mapping, even a single
store operation for the Employee class
now requires inserts or updates to two
different tables.
So even storing the data from a single class in a database can be a challenging exercise.
Then, he describes how relationships are different. At the object level model, you traverse objects via their relations. At the relational model level, you use foreign keys and joins (sometimes via a join table that doesn't even exist at the object model level).
Inheritance is another "problem" and can be "simulated" in various ways at the relational model level: you can map an entire hierarchy into a single table, you can map each concrete class to its own table, you can map each class to its own table.
In other words, there is no direct and unique correspondence between an object model and a relational model. Both rely on different paradigms and the fit is not perfect. The difference between both is known as the impedance mismatch, which is something ORM have to deal with (allowing the mapping between an object model and the many possible representations in a relation model). And this is what the whole section you're reading is about. This is also what you missed :)
I am just starting out with ADO.net Entity Framework I have mapped two tables together and receive the following error:
Error 1 Error 11010: Association End 'OperatorAccess' is not mapped. E:\Visual Studio\projects\Brandi II\Brandi II\Hospitals.edmx 390 11 Brandi II
Not sure what it is I am doing wrong.
I believe I can add some more clarity to the issue (learning as I go):
When I look at the Mapping details and look at the association, the column for operatoraccess table (from above) is blank and the drop down only includes field from the linked table.
The Entity Framework designer is terrible - I've had the same problem many times (and your problem too, Craig):
This happens when you have a many-to-one association which is improperly setup. They could very easily fix the designer to make this process simple; but instead, we have to put up with this crap.
To fix:
Click on the association, and go to the mapping details view.
Under association, click on Maps to <tablename>. Choose the table(s) which make up the many side of the relationship (ie. the table(s) which make up the *-side of the association in the designer)
Under Column, choose the table-columns which map to each entity-side Property. You get this error when one of those entries are blank.
I had the exact same problem and this is what I did to fix it.
Make sure you have an Entity Key set in your designer on the tables your making an association with. Also check that StoreGeneratedPattern is set to Identity for that Entity Key.
There's not a lot of information in your question, but, generally speaking, this means that there is an incompletely defined association. It could be that you have tried to map one table with a foreign key to another table, but have not mapped that other table. You can also get this error when you try to do table per type inheritance without carefully following the steps for implementing that feature.
Not sure of the answer, but I've just posted a similar question, which may at least help clarify the issue you are experiencing.
Defining an Entity Framework 1:1 association
I had to go back into the database itself and clarify the foreign key relationship
I had this problem in the case where I was creating both many to 0..1 and 0..1 to 0..1 associations. One entity needed associations to multiple tables, and that entity did not have foreign keys defined for those tables.
I had to do the table mappings step that is given in the accepted answer, but note that it wasn't only for many to many associations; it applied to all the types of associations I added for this entity.
In the Mapping Details view, I had to select the entity with the non-foreign key ID columns to the various tables. This is not always the "many" side of the relationship. Only there was I able to map the related entity property to the appropriate property in the original entity. Selecting the "destination" entity would not allow me to select the properties that I needed to, and the error would still exist.
So in short, I had to map using the table related to the entity that had the "non-foreign key" ID fields corresponding to the various entities' (and their tables') primary keys that I needed to associate.
Entity A
various other properties...
Id
ContactId
OrderId
etc.
Contact entity
Id
FirstName
LastName
etc.
In the mapping details, I selected Entity A's table. It then showed both ends of the association. I mapped its Entity A's Id property to its table's actual ID column (they had different names). I then mapped the Contact entity's Id field to the ContactId field on the A entity.
Simply select the many relationship table (*) from the Association>Edit Mapping & select the appropriate relationship