Core Data Programming Guide: Creating Managed Object Relationships
PHOTOGRAPH BY GREGG SEGAL NOVEMBER LOS ANGELES 45 CLASSIC LA PRIME NDS STREET SMART L.A.'S OLDEST THOROUGHFARE, dating. appears to be the crucial element of starvation, present solutions to this problem look towards newly-developed high-protein substances or protein additives. What does a fetched property look like? In the Core Data model editor, a fetched property is added just like an attribute or a relationship.
If that validation fails the domain instance will not be persisted to the database.
6 Object Relational Mapping (GORM)
By default, save will simply return null in this case, but if you would prefer it to throw an exception you can use the failOnError argument: Just remember that when you are saving domain instances that have been bound with data provided by the user, the likelihood of validation exceptions is quite high and you won't want those exceptions propagating to the end user. You can find out more about the subtleties of saving data in this article - a must read!NSManagedObject Subclasses - Beginning Core Data - catchsomeair.us
A common error that may occur is if you violate a database constraint, although this is normally down to a programming or schema error. The following example shows how to catch a DataIntegrityViolationException that is thrown when you violate the database constraints: If you really need to batch delete data you can use the executeUpdate method to do batch DML statements: The key part to remember is the belongsTo setting which controls which class "owns" a relationship.
If you do not define belongsTo then no cascades will happen and you will have to manually save each object except in the case of the one-to-many, in which case saves will cascade automatically if a new instance is in a hasMany collection. Here is an example: To understand this better take a look at the summaries below that describe the default behaviour of GORM with regards to specific associations.
Also read part 2 of the GORM Gotchas series of articles to get a deeper understanding of relationships and cascading. This is best explained by example: You can also use fetch: This works well for single-ended associations, but you need to be careful with one-to-manys.
Queries will work as you'd expect right up to the moment you add a limit to the number of results you want. At that point, you will likely end up with fewer results than you were expecting. The reason for this is quite technical but ultimately the problem arises from GORM using a left outer join. So, the recommendation is currently to use fetch: Be careful how and where you use eager loading because you could load your entire database into memory with too many eager associations.
Using Batch Fetching Although eager fetching is appropriate for some cases, it is not always desirable. If you made everything eager you could quite possibly load your entire database into memory resulting in performance and memory problems.
An alternative to eager fetching is to use batch fetching. You can configure Hibernate to lazily fetch results in "batches".
For example if you had an Airport that had 30 flights, if you didn't configure batch fetching you would get 1 query to fetch the Airport and then 30 queries to fetch each flight.
In the Core Data model editor, a fetched property is added just like an attribute or a relationship. Further, the predicate can use one of two variables to help configure it: By using this variable we can reference properties in the owning entity.
The property description includes key value pairs that are accessible from within the predicate. I have not run across a situation where I have used this variable.
The sharp edges There are unfortunately some concerns with fetched properties. Unlike a relationship, there is no way to pre-fetch a fetched property. Therefore, if you are going to fetch a large number of entities and then desire to access the fetched property for those properties, they are going to be fetched individually. This will drastically impact performance.
- Your Answer
- Table of Contents
- Sending feedback…
Fetched Properties are only fetched once per context without a reset. A fetched property always returns an array. Likewise, if you define a nonoptional to-many relationship from Employee to Address with a minimum count of 1, simply creating an instance of Employee does not create a new Address instance.
6 Object Relational Mapping (GORM) - Reference Documentation
Inverse Relationships Most object relationships are inherently bidirectional. If a Department has a to-many relationship to the Employees who work in a Department, there is an inverse relationship from an Employee to the Department that is to-one. 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 Weak Relationships Fetched Properties.
objective c - Fetched Properties v Relationships (Core Data - iPhone) - Stack Overflow
The recommended approach is to 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. Relationship Delete Rules A relationship's delete rule specifies what should happen if an attempt is made to delete the source object. Note the phrasing if an attempt is made. If a relationship's delete rule is set to Deny, it is possible that the source object will not be deleted.
Consider again a department's employees relationship and the effect of the different delete rules.
Deny If there is at least one object at the relationship destination employeesdo not delete the source object department.