I have situation where I dont want to add records to the relation table.
For example :
I have "TRIPS" entity and it has attribute for "LOCATION_ID", I am filling it when user creates a new TRIP and select a LOCATION from the LOCATIONS entity
In "LOCATIONS" entity I am allowing user to create locations and I am assigning a unique ID to each location.data will not be repeated here.
Is there any way to link the LOCATION_ID into LOCATIONS entity ,so when ever I access a trip(NSManagedObject) it automatically get LOCATIONS entity record (Object) ?
I mean automatically (Manually I can do that)
Thanks,
Raghu
If I understand correctly your question, you simply need to model differently your entities in the Core Data model, as follows. In your TRIPS entity, add LOCATIONS as a relationship, not as a property as you currently do. The relationship may either be to-one or to-many from TRIPS to LOCATIONS, depending on the constraints you want to enforce in your application, and to-one from LOCATIONS to TRIPS.
Once you do this, when you fetch objects from the TRIPS entity, they will also contain a LOCATIONS object (if you decide to use a to-one relationships) or a set of LOCATIONS objects (if you decide for the to-many relationship).
Related
I'm having a little trouble grasping CoreData relationships, i'm note sure which relationship type I should be using between my 2 entities or if my logic is correct.
1) "Person" Entity - attributes such as name, tel, address, country, etc...
2) "CountryList" - attributes such as countryName, countryLat, countryLong, etc..
The CountryList entity is pre populated on first run of the app to include all the countries in the world and their respected data.
Where i'm stuck is do I need a relationship between these two entities?
I will be allowing the user to select a country from the CountryList entity data and wish to store there selection in the country attribute for Person entity.
Do I just take the countryName from CountryList as a string and store it in country from Person? or can I make a relationship between them?
I know a user can only belong to 1 country but a country can have lots of users so is this a one to many relationship? Or is it many to many because lots of users can belong to a country but a country can have loads of users? Confused!
Could someone please enlighten me on this and point me in the right direction in what i should be doing in xcode.
Many Thanks in Advance
Matt
EDIT: Is this correct?
I have made the changes to Entity names etc and think I now have the relationship set correctly.
EDIT 2: Removed country attribute and renamed relationships
Firstly, your "CountryList" entity should be called "Country", since it represents only one country. The fact that you have many of those countries has nothing to do with its name.
After that, it seems just natural to use a relationship, one "Person" has one "Country", but one country can have many persons. Therefore, one-to-many relationship. Using a relationship will simplify many operations you might want to perform (i.e. access all the country information of one person, or get a list of all persons being in one particular country).
Oh, and this might help you understand relationships a bit better: There are no "many-to-many" relationships in CoreData per se. You always define a relation from a source to a target. So if you define a relation from Country to Person, this will be a one-to-many relationship. One country, many persons. You can then define a relationship from Person to Country, which would be a one-to-one relationship. One person, one country. If you defined this as an one-to-many relationship, you would end up with a de facto many-to-many relationship (because on person can have many countries and one country can have many persons). It's not as complex as it appears.
Now, after you've defined your two relationships, you can set them as each others "Inverse Relationship". Do it for one of the relationships, the other one will be set automatically. After you did that, CoreData will for example update a Person's country when you add the person to the country's list.
See https://developer.apple.com/library/mac/#documentation/Cocoa/Conceptual/CoreData/Articles/cdRelationships.html for further information.
CountryList should just be Country
Then you set a 'many to one' relationship between Person.county and Country
You are using Core Data so you must define relationship between Person and Country if you want to fetch person's country from database.
And in this relationship you may take one to one relationship. As One person will belong to one country only. Of Course a country will have many person but unless you want to show all people related to particular country you do not need one to many relationship..
In case you want to implement one to many relationship this tutorial link maybe helpful to you for understanding one to many relationship..
http://www.theappcodeblog.com/2011/09/29/iphone-app-development-tutorial-core-data-part-2-one-to-many-relationship/
Lets say I have 2 tables on my physical model, Receipt(ID, Location) and LineItem(ID, ReceiptID, ItemName) where a Receipt has multiple LineItems and ReceiptID is a Foreign Key to Receipt's ID.
I want to model these as a single table in my conceptual model, where I only see a table of LineItems with the Location included on each LineItem.
Every time I try to model this in the Entity Modeler, I get an error about how the Primary Key must be the same for every table being combined into the single conceptual entity.
Is this even possible to model using the entity framework?
Thanks!
No there is no way to model this directly. You must either create database view and map that view or import both entities and create QueryView in the model. In both cases resulting entity combining your two tables will become readonly and the only way to support CUD operations will be mapping stored procedures.
Let's say I have a table called "Event" which has a to-many relationship with table "Group" which has a to-many relationship with table "Person".
Many groups can be created which could reference the same person.
I have not added an inverse relationship yet from "Person" to "Group" since if a "Group" is deleted I do not want the associated "Person" to be deleted as it may be referenced somewhere else.
Should I just leave out the inverse relationship? The docs advise strongly against this but I think this many be an instance where it is the right way to go.
Any help is appreciated.
Unless you have a very good reason to do otherwise, you should always have an inverse relationship. It helps CoreData maintain the integrity of the object graph. The default delete rule for a relationship is "Nullify", which sounds like what you want. "Nullify" means that if you delete this object, the relationship attribute on the other end of this relationship gets nilled-out (for a to-one relationship) or has the value removed from the set (for a to-many relationship). The only way for the other end of the relationship to actually be deleted is if you change the delete rule to specify that.
tl;dr: Use inverse relationships, leave the Delete Rule at the default of "Nullify".
I have two entities: match and player , which I have set up with an intermediate table matchplayer as players can play multiple matches, and matches will have multiple players. I have views where I can add, amend and delete both matches and players separately, but I am confused about how to populate the matchplayer entity. I have set up the matchplayer entity with no attributes but with relationships to both the match and player entities.
I have a view managing match details, and from there it loads a tableview of all players where I can select the players I want to 'connect' to the match. But that's where I get stuck. What do I add to the matchplayer entity to store the relationship?
you don't need to create a separate entity just to maintain relationship.
What you can do is you maintain to many relationship from match entity to player entity.
Suppose name that relationship as playerInMatch. Now each managedObjectContext of match that you insert in match entity, you can insert multiple players to this managedObjectContext using addPlayerInMatch: method (i.e the accessor method that automatically gets generated when you created modal classes).
Now during retrival of a particular match , you can retrive a complete set of players of that match through its relationship.Using that you can delete or add any player to that set if you want to.
(I have briefly explained you the idea hoping you have a clear understanding of using core data relationship.If you are not perfectly clear with core data check out following tutorial:
http://cocoadevcentral.com/articles/000085.php
Please help an EF n00b design his database.
I have several companies that produce several products, so there's a many-to-many relationship between companies and products. I have an intermediate table, Company_Product, that relates them.
Each company/product combination has a unique SKU. For example Acme widgets have SKU 123, but Omega widgets have SKU 456. I added the SKU as a field in the Company_Product intermediate table.
EF generated a model with a 1:* relationship between the company and Company_Product tables, and a 1:* relationship between the product and Company_Product tables. I really want a : relationship between company and product. But, most importantly, there's no way to access the SKU directly from the model.
Do I need to put the SKU in its own table and write a join, or is there a better way?
I just tested this in a new VS2010 project (EFv4) to be sure, and here's what I found:
When your associative table in the middle (Company_Product) has ONLY the 2 foreign keys to the other tables (CompanyID and ProductID), then adding all 3 tables to the designer ends up modeling the many to many relationship. It doesn't even generate a class for the Company_Product table. Each Company has a Products collection, and each Product has a Companies collection.
However, if your associative table (Company_Product) has other fields (such as SKU, it's own Primary Key, or other descriptive fields like dates, descriptions, etc), then the EF modeler will create a separate class, and it does what you've already seen.
Having the class in the middle with 1:* relationships out to Company and Product is not a bad thing, and you can still get the data you want with some easy queries.
// Get all products for Company with ID = 1
var q =
from compProd in context.Company_Product
where compProd.CompanyID == 1
select compProd.Product;
True, it's not as easy to just navigate the relationships of the model, when you already have your entity objects loaded, for instance, but that's what a data layer is for. Encapsulate the queries that get the data you want. If you really want to get rid of that middle Company_Product class, and have the many-to-many directly represented in the class model, then you'll have to strip down the Company_Product table to contain only the 2 foreign keys, and get rid of the SKU.
Actually, I shouldn't say you HAVE to do that...you might be able to do some edits in the designer and set it up this way anyway. I'll give it a try and report back.
UPDATE
Keeping the SKU in the Company_Product table (meaning my EF model had 3 classes, not 2; it created the Company_Payload class, with a 1:* to the other 2 tables), I tried to add an association directly between Company and Product. The steps I followed were:
Right click on the Company class in the designer
Add > Association
Set "End" on the left to be Company (it should be already)
Set "End" on the right to Product
Change both multiplicities to "* (Many)"
The navigation properties should be named "Products" and "Companies"
Hit OK.
Right Click on the association in the model > click "Table Mapping"
Under "Add a table or view" select "Company_Product"
Map Company -> ID (on left) to CompanyID (on right)
Map Product -> ID (on left) to ProductID (on right)
But, it doesn't work. It gives this error:
Error 3025: Problem in mapping fragments starting at line 175:Must specify mapping for all key properties (Company_Product.SKU) of table Company_Product.
So that particular association is invalid, because it uses Company_Product as the table, but doesn't map the SKU field to anything.
Also, while I was researching this, I came across this "Best Practice" tidbit from the book Entity Framework 4.0 Recipies (note that for an association table with extra fields, besides to 2 FKs, they refer to the extra fields as the "payload". In your case, SKU is the payload in Company_Product).
Best Practice
Unfortunately, a project
that starts out with several,
payload-free, many-to-many
relationships often ends up with
several, payload-rich, many-to-many
relationships. Refactoring a model,
especially late in the development
cycle, to accommodate payloads in the
many-to-many relationships can be
tedious. Not only are additional
entities introduced, but the queries
and navigation patterns through the
relationships change as well. Some
developers argue that every
many-to-many relationship should start
off with some payload, typically a
synthetic key, so the inevitable
addition of more payload has
significantly less impact on the
project.
So here's the best practice.
If you have a payload-free,
many-to-many relationship and you
think there is some chance that it may
change over time to include a payload,
start with an extra identity column in
the link table. When you import the
tables into your model, you will get
two one-to-many relationships, which
means the code you write and the model
you have will be ready for any number
of additional payload columns that
come along as the project matures. The
cost of an additional integer identity
column is usually a pretty small price
to pay to keep the model more
flexible.
(From Chapter 2. Entity Data Modeling Fundamentals, 2.4. Modeling a Many-to-Many Relationship with a Payload)
Sounds like good advice. Especially since you already have a payload (SKU).
I would just like to add the following to Samuel's answer:
If you want to directly query from one side of a many-to-many relationship (with payload) to the other, you can use the following code (using the same example):
Company c = context.Companies.First();
IQueryable<Product> products = c.Company_Products.Select(cp => cp.Product);
The products variable would then be all Product records associated with the Company c record. If you would like to include the SKU for each of the products, you could use an anonymous class like so:
var productsWithSKU = c.Company_Products.Select(cp => new {
ProductID = cp.Product.ID,
Name = cp.Product.Name,
Price = cp.Product.Price,
SKU = cp.SKU
});
foreach (var
You can encapsulate the first query in a read-only property for simplicity like so:
public partial class Company
{
public property IQueryable<Product> Products
{
get { return Company_Products.Select(cp => cp.Product); }
}
}
You can't do that with the query that includes the SKU because you can't return anonymous types. You would have to have a definite class, which would typically be done by either adding a non-mapped property to the Product class or creating another class that inherits from Product that would add an SKU property. If you use an inherited class though, you will not be able to make changes to it and have it managed by EF - it would only be useful for display purposes.
Cheers. :)