How can I change the type of a Core Data entity column programmatically? For example, form String to Int 16. The entity can be assumed empty (no data row).
Firstly, it's not an SQL column, it's the attribute of an entity. Attributes have behaviors, columns in SQL do not.
In answer to your question, once a data model model has been used to write data to a persistent store file (sqlite store or other formats) it cannot altered programatically. If you change an attribute, you need to perform a version migration.
See: Core Data Model Versioning and Data Migration Programming Guide for details.
A little general advice: Core Data is not SQL. Entities are not tables. Objects are not rows. Attributes are not columns. Relationships are not joins. 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.
You can manipulate the attributes (including) type of an entity in an NSManagedObjectModel until it is used to initialize a persistent store coordinator. So, create the NSManagedObjectModel, mutate the attribute (not a column) type, then setup the Core Data stack as usual.
Of course, if you already have any data persisted using the original model, you'll have to perform a schema migration to update the data to the new type.
Related
I'm building my EF (v4.0.30319) data model from my SQL Server database. Each table has Created and Updated fields populated via database triggers.
This database is the backend of a ASP.NET Web API application and I recently discovered a problem. The problem is, since the Created and Updated fields are not populated in the model passed into the api endpoint, they are written to the database as NULL. This overwrites any values already in the database for those properties.
I discovered I can edit the EF data model and just delete those two columns from the entity. It works and the datetimes are not overwritten with NULL. But this leads to another, less serious but more annoying, problem... my data model has a bunch of tables that contain these properties and all the tables need to be updated by removing these two columns.
Is there a way to tell EF to ignore certain columns in entities across the entire data model without manually deleting them?
As far as I know, no. Generating the model from the database is going to always create all of the fields from the database table. Well, as long as it has a primary key it will.
It is possible to only update the fields that you want i.e. don't include the "Created" and "Updated" fields in your create and update methods. I'd have to say though, that I'd think it'd be better if those fields didn't even exist on the model at that point. You may at one point see those fields on the model and not remember that they won't get persisted to the DB.
You may want to look into just inserting the datetimes into those fields when you call your create() and update() methods. Then you could just ditch the triggers. You'd obviously want to use a class library for all of your database operations so this functionality would be in one place. To keep it nice and neat, you know?
Trying to understand Entity Framework. My approach is database first. However I would like to define other entites in the model that is closer to my business objects. I guess I could write queries in the db and include them in the model. But I would also like to define entirely new entities in the model though they would be based on underlying tables in the db. How do I do that - does anyone know a tutorial?
Regards
Bjørn
db Oldtimer, EF Newbie
Database first means that you have existing database and you can either create model by updating from database or manually. You can use wizard to create initial model and modify it manually to define new entities but you must not use update from database any more or some of your changes will be deleted. Also your custom modifications must follow EF mapping rules (for example it is not directly possible to map multiple entities to the same table except some more advanced mapping scenarios like splitting and inheritance) and some of them (custom queries) must be done directly in EDMX source (XML) because designer doesn't support them - this requires more complex knowledge of EF mapping and it will be definitely hard for newbie.
You can check specification of that XML. For entities mapped to custom queries you will have to use DefiningQuery element in SSDL part of EDMX.
I have a model object and the model object contains many variables and arrays. i need to save most of the values of model to database for future use. How can i achieve this? .
I created a table. Is i must insert each and every value individually?..Or is there any other way?
You may take a look at the tutorials here which illustrate how you could use Entity Framework within your application to map those models to SQL tables and perform queries.
Hello fellow stackoverflow family members?
I know it is un-efficient to create one extra sqlite table in iPhone CoreData system. (X)
Currently, My app has one sqlite table but there are preset data to users to no need to waste parse time. But if I adding new entity in current structure of SQLite table, it wipes up the whole preset data. I haven't tried to use immigrate method but I don't think it wouldn't be just adding a new entity on the table. I'm thinking it also wipes up previous preset data.
Current Architecture of SQLite file.
entity : A
attributes : contains data
I want to keep A with attributes still contain data but also add new entity : B.
entity : A
attributes : previous contained data
(PLUS+)
entity : B
attributes : new data
Do I need to create extra persistence set to separately store entity B (create another SQLite file and use as storage) or Is there possible way to add entity B in current SQLite with no changes in entity A?
Thank you.
To directly answer your question: Yes, you can reference multiple SQLite files in Core Data. You'll want to use a unique persistent store for each one.
I don't understand the rest of your question though, so it may be likely that creating multiple persistent stores is not what you want at all.
I know it is in efficient to create one extra sqlite table in iPhone CoreData system.
This is untrue. Creating multiple entities (aka, tables, but that is abstracted away) can lead to much more efficient queries.
Currently, My app has one sqlite table but there is no need to change preset sqlite table but want to add some other attribute.
Just add the attribute to your entity. If your app has already been released or you want to maintain existing data, you'll have to set up a migration and/or turn on lightweight migration.
So commit with new attribute and update coredata is not valid. Because it flash off current data set and need to spend another hours to type manually.
This didn't make any sense to me. Can you clarify?
I have an iPhone Core Data app with a pre-populated sqlite "baseline" database. Can I add a second smaller sqlite database with the same tables as my pre-populated "baseline" database but with additional / complementary data such that Core Data will happily union the data from both databases and, ultimately, present to me as if it was all a single data source?
Idea that I had is:
1) the "baseline" database never changes.
2) I can download the smaller "complementary" sqlite database for additional data as and when I need to (I'm assuming downloading sqlite database is allowed, please comment if otherwise).
3) Core Data is then able to union data from 1 & 2. I can then reference this unified data by calling my defined Core Data managed object model.
Hope this makes sense.
Thanks in advance.
Core Data is designed to handle multiple data files via the – addPersistentStoreWithType:configuration:URL:options:error: method. This will allow you to combine all of the data files together and then access them via a single NSManagedObjectContext.
Your only issue, and it may not even be an issue for you, is that the store files cannot directly reference each other. Therefore you will need to reference data between files "manually" via unique identifiers. However I suspect you are already aware of that limitation.
Manual Relationships
The idea is that when both objects in a "relationship" are in one model and one file, Core Data does its magic and handles all of the referential integrity for you. However when they are in different files and/or models this doesn't happen automatically anymore.
The solution to this issue is to use a fetched property that looks up some unique identifier to retrieve the entity (or entities) that you want to be on the other side of the relationship. This will give you a "weak" relationship between files.
One thing to note though when doing this. The fetched property does not get updated automatically when something changes. This means when data changes that would cause that relationship to change, your application will not be automatically made aware of it and you will need to request that property again to get the updated relationship information.
Hopefully that makes it a bit clearer.
Co-existance of fetched properties and relationships
They can definitely co-exist but realize that they are two separate properties. If you want your controller code to see them as one, then I would suggest building a subclass for that entity and then adding a convenience method in there that hits both the relationship and the fetched property and then rolls them up into one NSArray or NSSet before returning it back to your controller code.
You can attach the downloaded database with ATTACH DATABASE statement and operate with unions of tables.