It's been a while since I've had to work with relational databases (I've been avoiding it as much as I can in my personal projects, and at work we use an object database) so I'm fairly sure this is the correct way, but I wanted to make sure.
When modeling a relational database from an existing object hierarchy it is generally the case that non-primitive child properties should go in their own tables with a reference to the ID of the parent object (table-row thingy). It seems a little backwards from an OO standpoint (and obviously I realize that relational is way different from OO) but I guess it's the only way to represent one-to-many relationships (which feels like it should be more accurately described as many-to-one as the one parent doesn't reference the many children, it's many child entries that reference the one parent)
Yes.
In one-to-many relationships, the child points to the parent.
In many-to-many relationships (and one-to-many relationships with attributes, ternary relationships, etc.), you have relation tables.
In one-to-one relationships, you either merge the tables, or either entity can point to the other.
While this is by no means the only way, it certainly is the only sane way of representing relationships. All other methods I can think of break normalization (in fact, I can't even think of another 1NF method, let alone 3NF).
Related
I have been searching all over the internet and can't seem to find anything that specifically answers my question.
As far as I can tell, a simple association does not imply any form of life dependency.
A Composition is a whole-part relationship where the lives of the two classes are tied. For example, building and room. A room can not be created without a building, and if a building "dies", so does the room, and vice versa. BOTH are dependent on each other.
I do understand that aggregation is a weaker composition. For example, Car and Tires. But does aggregation imply dependence on the whole's side? Can the whole exist without the parts? Also, in aggregation, do the parts only belong to one whole?
I've found conflicting answers...
I have one more question. Can a simple association be a one-to-many relationship? For example, I am designing a prison management system, a PrisonBlock has Guards. If I say a guard is only assigned to one block. Their lives are obviously not dependent on each other. But there IS, however, a whole-part relationship. Or is there!? I'm confused. The way I see it, the Block (whole) has Guards (parts). Is this association or aggregation? And why? What would I have to change for it to become one or the other? Can an association even BE a one-to-many relationship!?
Aggregation is simply Has-a realtionship. In your example, Car should have tires . Because if it is not then that is not a Car. But Tires doesn't need to have car necessarily. Simply it is Has-a relationship.
2.Association is a relationship between two separate classes which can be of any type say one to one, one to may etc. It joins two entirely separate entities.
Aggregation is a special form of association which is a unidirectional one way relationship between classes (or entities)
THink like this:
If your prisonBlock class can exist without Guard class?
Or Guard class can exist without your prisonBlock class?
In compostion we create a opject which is defined in that class scope for example
class a:
b comObject = new b()
while in aggregation shows has a relation which means object has parent child kind of relationship but this do not means that when parent class die child also die because parent just make a deep copy
in case of association we only make a shallow copy and the differences between association and aggregation is that the related object can not have another parent
I have those four tables in database :
USER
id
PERMISSION
id
OBJECT
id
CONTEXT
id
Now the problem is that I want to link them to say that a user has one or many permissions on one or many objects depending of a context.. It looks simple but I can't find a way to avoid n-ary association..
Hope someone will be kind enough helping me to solve this problem.
Thanks in advance.
You may be looking for something like a WEAK ENTITY
Basically, a weak entity is a database entity which doesn't make sense on its own, but needs one (or more) foreign keys to assume a proper identity and a meaning.
This means that you're moving from an N-ary relationship to N binary relationships.
One possible approach is this: let's say that we call this weak entity Rules
Rules(id, user_id, permission_id, object_id, context_id /*other columns*/);
each of your strong entities has a relationship with the rules table. I don't like a lot this approach, but for small datasets it may work pretty well.
As a general note, though, I suggest you to think a bit more about your database model: are you absolutely, positively sure that all these 4 entities have a so strong relationship together? For example, does "Context" has influence on users, objects and permissions or just on permissions? Does an object exists at the same time across multiple contexts, or it makes sense to bind an object inside a specific context (the same concept of a variable scope)?
I have a class structure which is akin to a PurchaseOrder (parent) and PurchaseOrderLine (child) pattern, where the order lines will only be saved to the DB by saving the parent PurchaseOrder and will only ever be accessed via the parent too.
The DB has PurchaseOrderLine.PurchaseOrder set to not permit null values.
It seems from searching through the web that it is not possible to have a uni-directional association from PurchaseOrder via an IList property without having to have a property on the line pointing back when the child has a NOT NULL constraint for its PurchaseOrder column.
Is this really the case? It seems like one of the most basic things one would want to do with an ORM, I find it difficult to accept that a product as mature as NHibernate cannot handle such a basic scenario.
No it's not the case. Please see the example provided in the answer to this question: When to use inverse=false on NHibernate / Hibernate OneToMany relationships?
Well, it may be the case that you can't have unidirectional one-to-many relationship defined only on one side, but I'll argue with your statement that this is "one of the most basic things one would want to do with an ORM".
One of the most basic things would be to have unidirectional one-to-many defined only on many side - as it is natural for RDBM tables. And ORMs (despite the common misconception) are not intended (or able) to fully abstract domain model from underlying data source. Even if in some cases they can, the database side suffers from select N+1 problems or very ineffective queries.
Defining one-to-many at one side makes an impression that i.e. counting the collection is cheap. It is the case with plain object graphs, but not with NHibernate entities, as reading collection causes (at least one) call to the database. Eager fetching from one side is also not able to properly use database join mechanism in the way it's intended to be used (opposite to eager fetch from many side).
Even if I don't agree with a lot of arguments, I think it is useful to read some of the articles saying that "ORM is an anti-pattern", like this one. They helped me to leverage the way I think about ORMs and make me think about ORMs as a compromise between two not matching paradigms, but not the way to hide one behind another.
This can now be achieved in NH3 using the Not.KeyNullable()
this.HasMany(x => x.Actions)
.Access.BackingField()
.KeyColumn("[Application]")
.Not.KeyNullable()
.Cascade.AllDeleteOrphan();
Say you need to architect an app with an entity that can be associated with multiple other kinds of entities. For example, you have a Picture entity that can be associated with a Meal entity, a Person entity, a Boardroom entity, a Furniture entity, etc. I can think of a number of different ways to address this problem, but -- perhaps because I'm new to Core Data -- I'm not comfortable with any of them.
The most obvious approach that comes to mind is simply creating a relationship between Picture and each entity that supports associated pictures, but this seems sloppy since pictures will have multiple "null pointers."
Another possibility is creating a superentity -- Pictureable -- or something. Every entity that supports associated pictures would be a subentity of Pictureable, and Picture itself would have a one-to-one with Pictureable. I find this approach troubling because it can't be used more than once in the context of a project (since Core Data doesn't support multiple inheritance) AND the way Core Data seems to create one table for any given root entity -- assuming a SQLite backing -- has me afeard of grouping a whole bunch of disparate subentities under the umbrella of a common superentity (I realize that thinking along these lines may smack of premature optimization, so let me know if I'm being a ninny).
A third approach is to create a composite key for Picture that consists of a "type" and a "UID." Assuming every entity in my data model has a UID, I can use this key to derive an associated managed object from a Picture instance and vice versa. This approach worries me because it sounds like it might get slow when fetching en masse; it also doesn't feel native enough to me.
A fourth approach -- the one I'm leaning towards for the app I'm working on -- is creating subentities for both Picture and X (where X is either Meal, Person, Boardroom, etc.) and creating a one-to-one between both of those subentities. While this approach seems like the lesser of all evils, it still seems abstruse to my untrained eye, so I wonder if there's a better way.
Edit 1: In the last paragraph, I meant to say I'm leaning towards creating subentities just for Picture, not both Picture and X.
I think the best variations on this theme are (not necessarily in order):
Use separate entities for the pictures associated with Meal, Person, Boardroom, etc. Those entities might all have the same attributes, and they might in fact all be implemented using the same class. There's nothing wrong with that, and it makes it simple to have a bidirectional relationship between each kind of entity and the entity that stores its picture.
Make the picture an attribute of each of the entity types rather than a separate entity. This isn't a great plan with respect to efficiency if you're storing the actual picture data in the database, but it'd be fine if you store the image as a separate file and store the path to that file in an attribute. If the images or the number of records is small, it may not really be a problem even if you do store the image data in the database.
Use a single entity for all the pictures but omit the inverse relationship back to the associated entity. There's a helpful SO question that considers this, and the accepted answer links to the even more helpful Unidirectional Relationships section of the docs. This can be a nice solution to your problem if you don't need the picture->owner relationship, but you should understand the possible risk before you go down that road.
Give your picture entity separate relationships for each possible kind of owner, as you described in the first option you listed. If you'll need to be able to access all the pictures as a group and you need a relationship from the picture back to its owner, and if the number of possible owner entities is relatively small, this might be your best option even if it seems sloppy to have empty attributes.
As you noticed, when you use inheritance with your entities, all the sub-entities end up together in one big table. So, your fourth option (using sub-entities for each kind of picture) is similar under the hood to your first option.
Thinking more about this question, I'm inclined toward using entity inheritance to create subentities for the pictures associated with each type of owner entity. The Picture entity would store just the data that's associated with any picture. Each subentity, like MealPicture and PersonPicture, would add a relationship to it's own particular sort of owner. This way, you get bidirectional Meal<->MealPicture and Person<->PersonPicture relationships, and because each subentity inherits all the common Picture stuff you avoid the DRY violation that was bugging you. In short, you get most of the best parts of options 1 and 3 above. Under the hood, Core Data manages the pictures as in option 4 above, but in use each of the picture subentities only exposes a single relationship.
Just to expand a bit on Caleb's excellent summation...
I think it's important not to over emphasize the similarities between entities and classes. Both are abstractions that help define concrete objects but entities are very "lightweight" compared to classes. For one thing, entities don't have behaviors but just properties. For another, they exist purely to provide other concrete objects e.g. managed object context and persistent stores, a description of the data model so those concrete objects can piece everything together.
In fact, under the hood, there is no NSEntity class, there is only an NSEnitity***Description*** class. Entities are really just descriptions of how the objects in an object graph will fit together. As such, you really don't get all the overhead an inefficiency of multiplying classes when you multiply entities e.g. having a bunch of largely duplicate entities doesn't slow down the app, use more memory, interfere with method chains etc.
So, don't be afraid to use multiple seemingly redundant entities when that is the simplest solution. In Core Data, that is often the most elegant solution.
I am struggling with esactly this dilemma right now. I have many different entities in my model that can be "quantified". Say I have Apple, Pear, Farmer for all of those Entities, I need a AppleStack, PearStack, FarmerGroup, which are all just object+number. I need a generic approach to this because I want to support it in a model editor I am writing, so I decided I will define a ObjectValue abstract entity with attributes object, value. Then I will create child entities of ObjectValue and will subclass them and declare a valueEntity constant. this way I define it only once and I can write generic code that, for example, returns the possible values of the object relationship. Moreover if I need special attributes (and I actually do for a few of those) I can still add them in the child entities.
I have two items A and B, which have a uni directional one-to-one relationship. (A has one B)
In the database these are represented by ATable and BTable, and they are linked together by ABTable. (From the database setup it appears there is a many-to-many relationship but there is not, it was done this way for normalization reasons).
The problem is due to this setup, I have only been able to get NHibernate to map this as a many-to-many relationship between the entities. Is there anyway of making the entities have a one-to-one relationship?
The best I could think of is to leave its has a many to many relationship, and then have two properties on the A entity one that returns a List of B, which would satisfy the mapping and a second non-mapped property that would get the first B from the list, to satisfy my application. - but this seems un-eligant.
Are you sure you mean a one-to-one? I've had so many people ask for one-to-one's when they really mean many-to-one's.
Anyway, short of changing your schema, the easiest thing is what you suggested; however, to make it a little cleaner, you can make the collections private so you're only exposing the two properties that fetch the first item. You can see the various methods in Fluent NHibernate for mapping private properties on the wiki.
You might try combining the join-table with one-to-one mappings in various ways. A join-table mapping permits a single class to be persisted across more than one table which have a one-to-one relationship.