I have a core data model like so:
SALES_REP <--->> CUSTOMER <---->> PURCHASE_AGREEMENT <<------->> PRODUCTS
I can get the entity description for the Purchase Agreement and I can get the relationships (toCustomer and hasProducts)
[NSEntityDescription relationshipsByName];
but is it possible to get the Sales_Rep as well or do I have to pull that through the CUSTOMER entity?
Thanks
According to your diagram, SALES_REP isn't directly related to PURCHASE_AGREEMENT, it's only linked via CUSTOMER. That means SALES_REP doesn't know anything about PURCHASE_AGREEMENT on its own. So yes, you'll have to move on to the entity description for CUSTOMER and ask it for its relationshipsByName. I'm not sure what you're trying to do here, but it would be easy to recursively look up relationships on an entity, then its related entities, and their related entities, etc, until you don't find any new ones.
Related
I have some points on a map with associated informations contained by Core Data, to link the points on the map with the associated info, I would like to have an ID for each point, so in my entity, I would need some ID property, I have read that Core Data has it's own IDs for every managed object but I'm wondering wether or not it would be a good approach for me to directly use them or if I should create my own ID system ?
If you think I should create my OWN ID system, how would I do that ?
Thank you.
CoreData is not an relational database and you should avoid thinking about own ID’s. You may need them only for syncing purposes with external databases. For more precise answer, you should write how your model looks.
[edited after comment]
I don't see that you need any relations. Let's sat you have MapPoint entity with lat, and long properties. If there is only one user note, you just add another property to it like notes. If you have many informations (many records) stored with one MapPoint you need to add Notes entity with properties note and mappoint and make a relation between them. When you insert new Notes object into CoreData you set mappoint property to already existing MapPoint object (fetched after user tap).
Is there any way to get the name of a relationship given the from and to entities?
I have a fairly dynamic piece of code that needs to Associate or Disassociate entities. In order to call these functions, I need to know the name of the relationship. I am not sure how to determine the relationship name just given the two entities.
For example, my code might need to disassociate an account with a contact. How can I determine the relationship name of "account_primary_contact"?
In the interest of answering the question despite my not understanding why you would want to do this:
foreach (OneToManyRelationshipMetadata relationshipMetaData in primaryEntityMetadata.ManyToOneRelationships)
{
if (relationshipMetaData.ReferencingEntity == relatedEntity.LogicalName)
{
string relationshipName = relationshipMetaData.SchemaName;
//Do something with your relationship?
}
}
Where primaryEntityMetadata is of type EntityMetadata and ReferencingEntity is of type Entity. Then use similar logic for checking Many to One relationships.
Total newbie question now... Suffice to say, I have searched for a completely noddy explanation but have not found anything 'dumb' enough. The problem is...
I have created a core data stack in which I have a entity called 'Client' and an entity called 'Car'. It is a one-to-many relationship.
So far i have successfully created and fetched the client list using code from apple's tutorial. Once I select a client, I then push a new tableViewController which should list the Cars for that chosen client.
First question...
I am used to sql style database programming where if I wanted to add a car to a client, I would simply add a 'ClientID' tag to the 'Car' record thereby providing the relationship to a specific client. How do I do the equivalent in core data? My understanding from my reading is adding attributes to point to other entities isnt necessary - core data maintains this relationship for you without needing additional attributes in the entities.
Second question...
As and when I have created a 'car' entity and successfully linked it to a 'Client'. How to I create a fetch which will retrieve just THAT client's cars. I could alter the code from apple to fetch ALL cars but I don't know how to fetch cars associated with a given client. From my reading, I think I need to use predicates, but apples predicate documentation stands alone and does not give clear guidance on how to use it with core data
I realise how noddy this is, but I cant find an idiots guide anywhere...
Any help/code exmaples much appreciated.
OK, I have answered my own question. For those who have found my question and would like to know the answer, it is extremely simple...
Firstly, to create a 'Car' and associate it with a 'Client'. Firstly create a 'Car' as you normally would and simply add this line of code...
newCar.client = client;
This sets the 'client' relationship on the 'Car' record to the client in question.
Secondly, I had thought that if you had a client and needed to find their cars, you would need a new fetch. But not so! Simply use the following lines of code...
NSSet *cars = client.cars;
[self setCarsArray:[cars allObjects]];
The first line uses "client.cars" o follow the object graph to determine the cars this client has and populates them in an NSSet. The second line then populates a NSArray which is declared in the current viewcontroller which can be used to for display purposes.
Sorted!!
I am trying to do exactly same thing as post in NSFetchResultsController + sectionNameKeyPath + section order, i.e. basically use 2 tables, let's say Categories <-->> Events. Category table consists of category field only, while Event consists of name, dateTimestamp.
I defined relationship 'category' in Events table and try to use that relationship as sectionNameKeyPath when creating fetchedResultsController:
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:#"category.category" cacheName:#"Root"];
Finally, I pre-populated Category table with some categories upon loading of the app (and verified with .dump that table is populated correctly)
Yet, I simulator fails on:
return [[self.fetchedResultsController sections] count];
I did extensive search and most people either suggest using one of the fields in the table as sectionNameKeyPath (this works!) or transient property (works too!) However, I just want to use relationship as it seems very logical to me in this case where events belong to some categories and there could be categories without events. Am I wrong in my assumption that relationship can be used as sectionNameKeyPath? The original link at the top of the question suggests it works, but guy does not know why or how. Documentation is very weak on what can be used as sectionNameKeyPath, so any help will be highly appreciated.
A relationship gets you a pointer to a managed object. It seems logical, though, that the sectionNameKeyPath parameter should be a key path that leads to a string, since NSFetchedResultsSectionInfo's name property is a string. The fetched results controller will follow that key path for each fetched object and group the objects into sections based on what they return for that key path, and it'll also use those strings as the names of their respective sections. You can't use a managed object for the name -- you have to use some string property of the managed object.
So, your Category entity must have an attribute that distinguishes one category from another, right? Use that as the key path and (as you've seen) everything will work out.
BTW, I think it's useful to try to get out of the database (rows/fields) mindset and try to think in object-oriented terms like entity and attribute. A big selling point of Core Data is that it provides an abstraction layer that hides the storage mechanism. Thinking in terms of tables is like thinking about blocks and sectors when you're reading or writing a file.
Caleb, thank you for your answer. I do believe my understanding was wrong to some degree. What I had was an entity Category and entity Event. Category has a string field 'category', thus 'category.category' path (first 'category' is relationship in the Event entity)
What I did not take in account, though, is that if there are no events, fetchresultscontroller cannot fetch anything (similar to 'left join')
What I wanted is to show categories even if there are no events. Relationship 'category' will not return anything in this case as there is nothing to return/sort/categorize.
What I had to do (wrong or right - not sure yet) is to treat [managed] object created from Category entity as a separate object in case there are no events and place in the table. When there is one event per category, I can switch to the original method of [automatic] showing events sorted by categories.
This is interesting issue of starting point (empty entities with relationships) where I feel core data is more confusing than traditional relationship database. I also believe that's why all books/articles/reports carefully stay away from this topic. In other words, I could not find analog of "left join" in core data. May be I am wrong because I am relatively new to all this. Below is the description of the entities:
Category <-->> Event
Category - parent
Category.category - attribute of type String
Category.event - relationship to Event entity
Event - child
Event.name - attribute of type String
Event.category - relationship to Category entity
Each event belongs to one category. Category may have multiple events.
Categories should be shown even if there are no events for this category.
I was trying to put Events under fetchresultscontroller. May be I should switch to Category first and then calculate cell based on category.event relationship, not the other way around - did not try that yet.
We're working on a new project (re-writing existing app), and I'm running into problems with my domain model / repository design.
Here is a (simplified) version of two key portions in our domain model:
As you can see, I have an abstract concept of a Post, which can be things like a Review, a Discussion, a Photo, a Video, etc. Posts can also have comments.
I also have a abstract concept of a Location, which are obviously things like Streets, Cities, Neighbourhoods, etc.
Now, this naturally looked to me as two clear aggregate roots.
So I created two repositories, one called PostRepository, and another called LocationRepository.
This was all working fine, I can add/get any type of Post (or comment), and add/get any type of Location via one of these two repositories.
But now im in the scenario of a "landing page" for a City (for example).
On this page, I need to basically show "all posts for this location".
How is that defined? Well, a Post can be (optionally) tagged at a Location. Implementation detail, so I don't want to go too deep into data (as that's not what DDD is about), but essentially there is geospatial intelligence to work out which posts are contained in a particular location by the shape file of the location, and the latitude/longitude of the tagged Post.
But how can I retrieve this information without crossing the boundaries?
Which repository do I use? Do I need a new one?
If it matters (or for the curious), this is a web application (ASP.NET MVC), with a SQL Server 2008 database and Entity Framework 4.0.
If you need any clarification, let me know.
EDIT
We currently use a modified version of the Specification pattern in order to retrieve domain models.
For example, this is the code in our BLL to retrieve all Review's where Score >= 4:
var reviews = postRepository // GenericRepository<Post>
.Find() // IQueryable<Post>
.OfType<Review>() // IQueryable<Review>
.Where(x => x.Score >= 4)
.ToList(); // List<Review>
But now I need some code like this:
var reviews = postRepository
.Find()
.OfType<Review>()
.Where( //lat long, or Locations FK )
.ToList();
The problem is I don't know how to do the above query without adding an intermediary join-entity (LocationPost - as it's a many to many), and add a FK to the Post domain model to that.
But by doing that, I am crossing the aggregate boundaries - aren't I?
Why is this a problem?
According to Evans in his book, one AR may very well reference another AR.
(You may not however reference a child element in an AR from another AR)
Also, are locations really aggregate roots?
The definition of an aggregate root is that it acts as a boundary of concistency.
Does that fit the definition of a location?
I'd say a location is a value object.
There are pretty much two camps here regarding repositories and AR associations:
One that says that all aggregate roots have to be fetched through their respective repository, and AR's should use soft relations, eg ID's between them
And one that says that aggregate roots may very well fetch other associated aggregate roots and that a repository is merely a way to find aggregate roots.
I would bind post to the location at creation time so that for each location I can get (through a repository) a list of associated posts. It would look like this:
Creation:
var p = new Post(latitude, longitude);
var locations = locationRepository.FindByCoordinates(latitude, longitude);
foreach (var l in locations)
{
l.AssociatePost(p);
}
session.Save(p);
Retrieval:
var associatedPosts = postRepository.FindByLocation(locationId);
foreach (var p in associatedPosts)
{
Display(p);
}
Under the hood, the association between posts and location would be implemented as a many-to-many table relationship. There is one problem with this solution: adding a new location requires to scan all the posts and assign them to the new location (if applicable).
Hope that helps.
Let's say you used the Specification pattern, could you build a Post Specification using a Location object? Then you just pass the Specification to your Post Repository, and get back the result.