Let's say there is an entity called Product, which is connected to Shop (a shop has several products). However, there is also an entity called SpecialWebOffers, which also has a number of Products (which are not in any Shop).
How do I connect both Shop and SpecialOffers to Product. Is it OK to have two relationships in Product - one to Shop and one to SpecialWebOffers?
It's very common for an entity to have multiple relationships. If I am understanding your question correctly then the answer is yes.
Related
I'm trying to get my head around NoSQL and Google Cloud Datastore, and I don't know how to chose between two different options for storing data.
I have a list of orders, and every order is for an unspecified number of products. What are the pros/cons of storing the product list as an array property for the order entity vs having product child entities for each order parent?
Firstly, be well aware of the distinction between the 2 possible approaches of implementing a relationship between entities:
one entity can contain a Key type property pointing to another entity (which might or might not exist!) - this is a functional relationship only, not one at the datastore level
having the 2 datastore entities in a parent-child (ancestry) relationship, inside the same datastore entity group.
Using the 2nd one has scalability implications, see also:
Ancestor relation in datastore
E-commerce Product Categories in Google App Engine (Python)
As for storing a list as an array property vs as separate entities, see Creating your own activity logging in GAE/P (where repeated properties is just how array properties are called in the ndb client library context).
If models in Master Data Services are representations of subject areas (such as customer, product, etc.), is it possible to have a domain-based attribute that uses a different model as it's source?
For example, if creating a customer model and defining an entity called Customers, this entity will have address information for the customer. City, State/Province, Country, etc.
But rather than creating an entity for CustomerCountry, can I use a Geography model that contains a Country entity and link to that? Then I can also use the Geography.Country entity for Vendors and Employees.
No, you cannot cross model boundaries - the primary reason is that a model is the unit of versioning. There are two workarounds:
Duplicate the entities in question - i.e. copy the geography entities into your customer model
Reference an entity by code - i.e. add a CITY_CODE to your customer entity
Neither of these are great options 1) is unwieldy and 2) doesn't maintain automatic integrity.
Personally I use option 2) where I can with some additional logic/business rules outside of MDS to ensure that codes match. This is easier with codes that are global and do not change such as country and city codes.
This seems quite old but this is now possible in 2016 and is accomplished though Entity Synch. The idea is you can synch your entity across models. There are two ways to run the synch, either on demand or automatically.
See https://learn.microsoft.com/en-us/sql/master-data-services/entity-sync-relationship-master-data-services for more details
Ok, so I am new to entity framework...
I have an existing SQL database with some 500 tables in it, and we are in the process of considering a move from Linq->SQL to Entity Framework as our main data access layer. We also want to consider more of a domain driven design approach with separate data contexts managing key areas of the application ( i.e. Sales, Marketing, Jobs, Shipping etc. etc. ).
If we take a common entity such as "Customer", this appears in more than one model. I have two models in my sample app so far. Entity Framework is clever enough to create only one customer class ( we are using the default Poco T4 templates for class generation ), however when I try and run the project I get the following error "Multiple types with the name 'Customer' exist in the EdmItemCollection in different namespaces. Convention based mapping requires unique names without regard to namespace in the EdmItemCollection".
So am I right in thinking that Entity Framework does not allow "Customer" to exist in more than one model ? If I really want customer appearing in more than one model, do I have to start creating different versions of the customer class to deal with it ?
Apologies in advance if this is a dumb question, but I am relatively new to EF.
Thanks...
You said that you are creating DDD with bounded context. In bounded context, you create more than one context with one or more related entities in it. Why do you want to create more than one model with the same name?
Check the Julie Lerman's link for reference:
http://msdn.microsoft.com/en-us/magazine/jj883952.aspx
Sorry if I am out of context. But, in my experience in such a scenario, we have to create two different context such as "MarketingModelContext" and SalesModelContext. MarketingModelContext will have all the dbsets related to marketingmodel along with customer entity. In the same way, SalesModelContext will have all the dbsets related to SalesModel along with customer entity. In this way, you will be creating only one customer entity or POCO which can be used by two contexts independently. This is known as bounded contexts as Julie Lerman calls it. It will help you in separation of context, concerns and helps you with better performance as only required context(fewer entities) can be loaded. The above article will help you with this.
Hope I have answered your query.
Say you have a couple of Core Data entities .. Student and Exam. These two are initially filled with data from two xml files downloaded from the web.
Now, Students and Exams are separate things... initially there are no connections between them. But after filling out these two entities, I might want to connect certain students to certain exams. Or I might want make all students take a particular exam. But I still want to be able to treat Exams as independent things, which might have no students connected.
I'm unsure how to do this with Core Data. In the data model, you either have a relationship or yo don't. Should I have two different entities for Exam... one for independent exams, and one connected to the student which can be built up from the other Exam enitity?
No, you should not make two entity types.
Just because you have a relationship between two kinds of entities doesn't mean you can't create an object where that relationship is nil.
So, assuming you have a many-to-many relationship between Student and Exam, you might create a new exam by doing something like:
Exam *newExam = [NSEntityDescription
insertNewObjectForEntityForName:#"Exam"
inManagedObjectContext:context];
newExam.course = #"CS 101";
newExam.description = #"Midterm";
You might then later establish a relationship between a student and that exam like:
[newExam.students addObject:aStudent];
(where students is the name of the relationship between Exam and Student
I think you should have a relationship between the two entities (exam and student) but mark it as optional.
I have a database with a table for active orders and one for inactive orders. I would like to model this in Entity Framework as one entity called Orders. I also need a way to determine if an order in this collection is active or not, preferably by having a status property on the entity that is set according to what table it is in. Is there anyway to do this using Entity Framework 1. What about in Entity Framework 4?
You could create a view in your DB and generate the entity from that.
Take a look at the Table Per Concrete Type inheritance.
It is described here in ADO.NET Team Blog.
I think this is what you are looking for: How to: Define a Model with Multiple Entity Sets per Type (Entity Framework)
"The Entity Data Model (EDM) allows an entity type to be included by multiple entity sets within a single entity container or for an entity type to be included in entity sets in multiple entity containers. Defining multiple entity sets per type (MEST) allows users to streamline their code when databases have partitioning or other such scenarios where multiple tables have the same structure."
If I am understanding you correctly both active and inactive orders would share the same properties (for example: both would have a decimal "amount" property) if this is the case then in EF 1, I am pretty certain this is not possible. I think you will have to fall back to Mapping your entities to a POCO Orders object.
A good way to do one entity that shares multiple tables is to use Entity Splitting. MSDN has a very simple tutorial that walks you through the process which is very easy, however, you may need to reshape your data model: http://msdn.microsoft.com/en-us/data/jj715646.aspx