I am new to the core data and am loving it so far, I just have a question on to-many relations and their inverses. I'm trying to create something where each unit can convert to many other units so each unit can point to many converters that point to just one other unit. The image below works perfectly, but I know that core data wants them to be inverse and lets me know. When I try to select this it eliminates the functionality I desire.
I have tried creating a new relationship on each entity to act like an inverse but it fails, I can get a set of null objects.
Basically I am trying to create a graph-like structure in core data.
Is it possible to get this functionality while making core data happy with supplying inverses?
You should be able to make inverses by adding a relationship to each entity. The Converter entity would get a ConvertsFrom to-one relationship which is inverse to the Converters relationship, and Unit would get a ConvertedBy to-many relationship (since it can be converted by many converters) which is inverse to the ConvertsTo relationship.
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I’m using JPA 2.1 and I have an entity object called Encounter that has one-to-one relationships with many other different objects of varying sizes (call them satellite objects). I would like these satellite objects to behave like value objects and be automatically replaced when a new instance of Encounter is saved. I have read many articles that show how to achieve this behavior by making the satellite objects embeddable objects instead of entities in a one-to-one relationship. That would work fine except that for any one “encounter” only a few of the large number satellite objects will be needed. Embedding them in a single table would waste a lot of table space and make the Encounter table more complex than necessary.
So is there a way to make an Entity in a one-to-one relationship behave like a value object without writing a lot of code to check if it exist, then delete it, then persist the new one?
I have a domain model architecture in which my domain/business objects were created based on the problem domain and independent of any knowledge of the physical data model or persistence structures. So far I'm on track because it's perfectly acceptable and often the case that there is an impedance mismatch between the domain model and the data model. A DBA created the database for getting the data they required, but it does not encapsulate the applications entire domain model or design.
The result - I have my own set of domain model objects. However all of the fields that need to be persisted do exist somewhere or another within my domain model, but not necessarily in the shape that my auto generated .edmx POCO entities have them. So I have all the data, it's just not in the perfect shape exactly like the tables in which auto generated POCO entities are created from.
I have seen a few posts on this topic like converting POCO entity to business entity and Entity Framework 4 with Existing Domain Model that make statements like the following:
"Create the entities in your entity data model with the same names as
your domain classes. The entity properties should also have the same
names and types as in the domain classes"
What!? No way, why should I have to make my domain model be reshaped to POCOs that are modeled exactly after the data model / table structure in the database? For example - in my case of having 5 given properties, 2 might be in class 'A' and 3 in class 'B', whereas a auto generated POCO class has all 5 in its own class 'A'.
This is the entire point, I want separation of my object model and data model but yet still use an ORM like EF 5.0 to map in between them. I do not want to have to create and shape classes and properties named as such in the data model.
Right now my .edmx in EF 5.0 is generating the POCO classes for me, but my question is how to dissolve these and rewire everything to my domain objects that contain all this data but just in a different shape?
By the way any solution proposed using a Code First approach is not an option so please do not offer this. I need some guidance or a tutorial (best) using EF5 (if possible because EF4 examples are always inheriting POCOs from ObjectContext) with wiring up my own business objects to the .edmx.
Any help or guidance is appreciated, thanks!
This sounds like exactly the use case of Entity Framework. I am making a few assumptions here. First that when you make this statement:
"I have a domain model architecture in which my domain/business objects were created based on the problem domain and independent of any knowledge of the physical data model or persistence structures."
That you mean this domain was created in the EF designer? But then you say:
"However all of the fields that need to be persisted do exist somewhere or another within my domain model, but not necessarily in the shape that my auto generated .edmx POCO entities have them."
This sounds to me like my first assumption is incorrect.
Next, you dismiss code first? If your domain model/business objects are code based and you want to persist them to a relational database, that is exactly the use case for code first. You have the code, now you need to create your DbContext and map it to your physical model.
However you dismiss that... so some thoughts:
If you have a domain model of code based business objects and you have an EDMX that is used for other things I think you would want to create a repository layer that uses something like auto mapper or manual projections to query your Entities and return your business objects.
If you have a domain model of code based business objects and you have an EDMX that is not used for other things other than persisting your business objects I would say that you need to express your domain in an EDMX and then map it onto your existing database. This is really the use case for an ORM. Having two domain models and mapping from one model to the other where one model matches your domain and one matches your database is adding an extra un-needed layer of plumbing.
The latter approach above is what is called "Model First" in EF parlance. There are several articles written about it although the bulk of them just generate the db from the model. You would not do that step, rather you would map your entities onto your existing database.
The basic steps for this are to "update from the database" not selecting any of the db objects (or entities would be created). Or, you can take your exiting .edmx in the designer (which is sounds like you have) and modify the entities to match your business domain. Or just delete all the entities in your EDMX model, create your entities as you want them, and then map them all.
Here is a jing I made where I use the EF Designer to bring in the model store (the only way to do this is to allow it to generate entities) and then delete the entities allowing the Store information to stay by clicking NO when it asks if you want to delete the table info.
http://screencast.com/t/8eiPg2kcp
I didn't add the POCO generator to this, but if I did it would generate the Entities in the designer as POCO classes.
The statement quoted above is not suggesting that you rewrite your domain objects to match your pocos, it is suggesting that you update the edmx to match your domain model.
In your example you could create an entity in your edmx that maps all 5 properties from both tables and EF will manage the mapping to and from the single generated Poco onto your tables.
Of course this means that you then have duplicate domain objects and pocos, meaning you would either have to manually convert your domain objects to pocos when interacting with EF,
or you could define your domain data objects as interfaces and provide partial implementations of the pocos that essentially identify each poco as being a concrete implementation of a domain object.
There are probably several other ways to skin this particular cat, but I don't think that you can provide predefined c# objects for use in an edmx.
One approach might be to select into a ViewModel (suited to your particular business logic) and automatically map some data from the context into it like so https://stackoverflow.com/a/8588843/201648. This uses AutoMapper to map entity properties from an EF context into a ViewModel. This won't do everything for you, but it might make life a bit easier. If you're unhappy with the way this occurs automatically, you can configure AutoMapper to do things a bit differently (see Projection) - https://github.com/AutoMapper/AutoMapper/wiki/Projection
You might know this already, but its also possible to automatically generate POCOs from your EDMX using t4 - http://visualstudiogallery.msdn.microsoft.com/72a60b14-1581-4b9b-89f2-846072eff19d. If you define the templates to generate partial classes, you can then have another partial class with your custom properties for that POCO. That way you can have most properties automatically populated, but have other custom properties which you populate with custom rules from your context/repository. This takes a lot of the monotony out of generating these, and you can then focus on reshaping the data using the above technique.
If you're seriously unhappy with both, you could always map a stored procedure to get the exact field names that you want automatically without needing to stuff around. This will of course affect how you work with the data, but I have done it before for optimisation purposes/where a procedure already existed that did exactly what I wanted. See http://msdn.microsoft.com/en-us/data/gg699321.aspx
I asked this question earlier, but I'm missing one important thing. I have a NoteObject entity and every note, without Core Data, just has an arrayOfTags (which is an array of NSStrings). I decided to use to-many relationships to store the tags instead of an array. So I created a new "Tags" entity, and set up a to-many relationship from my NoteObject to Tags. This works great when every tag is related to only one note, but I'm unsure how I would go about linking one tag with multiple notes. How would I set up that relationship?
In you data modeler just make the Tags -> Notes relationship a to-many relationship. If you have already generated the Entity subclasses you will have to redo those. You will also have to be careful when creating tags to ensure uniqueness, but other than that it should be fairly straightforward. myTag.notes should work just as well as myNote.tags.
I have two objects, a Trip and a Place. A Trip represents a journey from one Place to another Place, ie. a Trip needs a fromPlace and a toPlace. So, this is a 1-to-2 relationship, but I need to know which is the "from" and which is the "to". I am not sure how to model this in Core Data. I have created two entities (Trip, Place), and now I want to setup the relationship(s) so I have a fromPlace and a toPlace. Do I need to add an extra field on the Place entity called isFrom, or similar?
If this was in a database, I would just have a id column on the Place table, and then two columns in the Trip table - fromPlaceId and toPlaceId. How do I achieve something similar in Core Data?
Do I need to add an extra field on the Place entity called isFrom, or similar?
Yes. It's better for you not to think of Core Data as a wrapper around a database; the database intuition sometimes gets in the way.
Don't first think in terms of database and then try to translate it into Core Data. While you're learning how to use Core Data, just think of it as a system of objects which can be saved into a file and persist between two launches of the app.
Then, from the point of view of object-oriented programming, you have a class Trip which has two instance variables fromPlace and toPlace of class Place.
You want to make it persist on a file. So you create an entity Trip which has two relations fromPlace and toPlace, both of which is of entity Place. That's it!
In more detail, fromPlace and toPlace in Trip are both to-one relationships. In Place, you make two to-many relationships, say tripsStartingHere and tripsEndingHere. Then you set tripsStartingHere as the inverse of fromPlace, and tripsEndingHere as the inverse of toPlace.
I have a Core Data object called Car that has a one-to-many relationship with another Core Data object called Claims (i.e., one Car has many Claims). I've created the entities in the data model editor, and am able to list, edit, and work with the Car objects.
What I can't figure out is this: given a Car, how to get an array of all related Claim objects. I think it requires a fetch request, but how does it work?
Thanks!
If the entities are related, you'll want to add a relationship to your models. In core data relationships are two-way, so Car would have a to-many relationship called "claims" and Claim would have a to-one relationship called "car".
Having these relationships in place will give an instance of Car an NSSet of claim objects.