ios - Core Data relationship to same entity - Stack Overflow
what is the difference between Entity, classification and relationship in perspective in some way are classified with same classification. E.g. Sensitive information will reside in several tables in several database in a data warehouse. Design & Architecture · Governance & Lifecycle · Hadoop Core. Hello, i'm new to Swift Programming and I'm struggling with Core Data a bit. My Data Model looks like the following: I have Items with multiple. Adding Core Data entity relationships: lightweight vs heavyweight migration. It's time What you'll see is well, exactly what you saw before: the same list of commits. . Learn about value types, functional programming, and protocol- oriented.
This is the target or the destination class of the relation. The answer to the question: Is the destination a single object or not? If yes, the relation is of type to-one, otherwise it is a to-many. The definition of the inverse function. It is pretty rare to find a domain where this is not needed. It is also a sort of logical necessity: In your example, a department can have more than one employee, so this is a to-many relation.05 Core Data Relationships - Fetching Categories - Expenses with Categories App
As a general rule, a one-to-many relation has a many-to-one inverse. In case you want to define a many-to-many relationship, you simply define one relation as to-many and its inverse as a to-many as well.
Make sure you define an inverse for each relationship, since Core Data exploits this information to check the consistency of the object graph whenever a change is made. This defines the behavior of the application when the source object of a relationship is deleted.
For the delete rule in 5 above, there are four possible values: Nullify is the simplest option. They just keep thinking they have not been fired: If you select cascade as the delete rule, then when you delete the source object it also deletes the destination object s.
Adding Core Data entity relationships: lightweight vs heavyweight migration
Such a rule is appropriate only if you want to close a department and fire all of its employees as well. In this case it is enough to set the delete rule for department to cascade and delete that department record. Deny, on the other hand, prevents accidental deletions. Delete rules have to be specified for both sides of a relationship, from employee to department and vice versa.
Each domain implements its own business logic, so there is no general recipe for setting delete rules. Just remember to pay attention when using the cascade rule, since it could result in unexpected consequences. To maximize the performance of your application, remember this when you devise your data model and try to use relationships only if necessary. The first step is to add a new entity. The delete rule is the default, nullify 4. As above, this is a to-many relationship 3 with a delete rule of nullify.
A new class, named Tag, will pop up in your project tree.
Sometimes, quite often, in fact: If this happens to you, select one set of instances and delete them, but choose to remove references rather than to trash the files. At this point, you have changed the Core Data model, so your app will not be compatible with the old model on your device. This new view controller will facilitate the creation of new tags and associating them to a bank details object.
Remember to check the box to create the accompanying XIB file. Finally, you add a method to initialize the component with an instance of details. Add the code for it as follows to the end of the file but before the final end: Replace the existing viewDidLoad with the following: You need this to show a tag as picked by means of a tick in the table view. You also set up a navigation item to add new tags.
Add the following below viewDidLoad: Now add the following code to the end of the file: The code above will display an alert asking the user to insert a new tag: In such a case, instead of implementing the change protocols to the table, you fetch the result again and reload the table view for the sake of simplicity.
YES]; if [pickedTags containsObject: Next implement this callback: At the bottom of the method implementation, add this code: As a final touch, make the label backgrounds gray to show their tappable area. Add this to the end of viewDidLoad: Build and run your application and test it. Add a new bank record. Tap the tags label will be empty the first time.
Note 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.
Among other features, each property has a name and a type. Core Data does track changes you make to transient properties, so they are recorded for undo operations. You use transient properties for a variety of purposes, including keeping calculated values and derived values. Note If you undo a change to a transient property that uses nonmodeled information, Core Data does not invoke your set accessor with the old value — it simply updates the snapshot information.
Swift Core Data Relationship to Same Entity
You can specify that an attribute is optional—that is, it is not required to have a value. In general, however, avoid doing so, especially for numeric values. Typically, you can get better results using a mandatory attribute with a default value—defined in the attribute—of 0. Moreover, NULL in a database is not equivalent to an empty string or empty data blob. Relationships and Fetched Properties To define a relationship, select it in the Core Data model editor, and specify values in the Relationship pane of the Data Model inspector; Relationship in the Data Model inspector.
Core Data Programming Guide: Creating Managed Object Relationships
Figure Relationship pane in the Data Model inspector Core Data supports to-one and to-many relationships, and fetched properties. Fetched properties represent weak, one-way relationships. The Type pop-up menu defines whether the relationship is a to-one type relationship or a to-many type relationship.
Relationships are defined from one direction at a time. To create a many-to-many relationship, you would need to create two to-many relationships and then set them up as inverses of each other.
The Destination pop-up menu defines what object or objects is returned when the relationship is accessed in code. If the relationship is defined as to-one, a single object or nil if the relationship can be optional is returned.
Deny If there is at least one object at the relationship destination employeesdo not delete the source object department.
For example, if you want to remove a department, you must ensure that all the employees in that department are first transferred elsewhere; otherwise, the department cannot be deleted. Nullify Remove the relationship between the objects, but do not delete either object.
This only makes sense if the department relationship for an employee is optional, or if you ensure that you set a new department for each of the employees before the next save operation. Cascade Delete the objects at the destination of the relationship when you delete the source. For example, if you delete a department, fire all the employees in that department at the same time.
No Action Do nothing to the object at the destination of the relationship. For example, if you delete a department, leave all the employees as they are, even if they still believe they belong to that department. It should be clear that the first three of these rules are useful in different circumstances. For any given relationship, it is up to you to choose which is most appropriate, depending on the business logic. It is less obvious why the No Action rule might be of use, because if you use it, it is possible to leave the object graph in an inconsistent state employees having a relationship to a deleted department.
If you use the No Action rule, it is up to you to ensure that the consistency of the object graph is maintained.