I was interested in storing the history of properties which contains two entites - properties and owners. Right now I am not sure what approach to take and needed some help.
I was thinking of creating an associative entity and so this would have its identity a combination of property and owner but the textbook comes up with this solution instead:
Solution
What's the difference between the solution above and an associative entity?
The owns table in the given solution would be called an associative entity set in the network data model. This data model supports only one-to-one and one-to-many binary relationships, and resolves many-to-many binary relationships as well as ternary and higher relationships into an associative entity set with binary one-to-many relationships.
However, in the entity-relationship model, the owns table represents a many-to-many relationship relation. The ER model directly supports many-to-many binary relationships as well as ternary and higher relationships, and uses "associative entities" to refer only to relationships which are the subjects of other relationships.
Related
I'm designing an ER diagram for a social network and recently I got involved in an argument with my colleagues whether this part is right or wrong
ER DIAGRAM PROBLEM
Where Faqet(Pages) is connected with Shfrytezuesi(User) using three actions, pelqen is for storing likes, krijon faqe to know who created the page, and udheheq to store all page admins, so my question is
is this design wrong?
Can two tables be linked with more than one action, this is where I'm not certain
It's perfectly valid to have any number of relationships between any number of entity sets. My only concern with the diagram is that multiple role lines below Shfrytezuesi are merged into one - I recommend keeping them distinct.
Note that in the entity-relationship model, we don't link tables. That idea comes from the old network data model, in which rows represented entities, tables represented entity sets, and links between rows/tables represented relationships.
One disadvantage with that model is that it supports only directed binary relationships - many-to-many binary, ternary and higher relationships and relationships with attributes all required associative entities to be introduced. However, three binary relationships aren't equivalent to a ternary relationship, and not all relationships can be represented in binary data models.
The ER model supports n-ary relationships and attributes on relationships. Entity sets are represented by their primary keys and relationships by combinations of entity keys. Entity sets plus attributes form entity relations, relationship sets plus attributes form entity relations. These relations get mapped to tables. In practice, tables with the same primary keys get combined to reduce the number of tables, which means one-to-one and one-to-many relationships get combined into the relations for one of their associated entity sets.
Regardless of how tables are combined, attributes and relationships are represented by sets of columns. For example, based on your diagram, Pelqen would be represented as (FID PK, SID) (assuming SID is the primary key of Shfrytezuesi). These columns might have different names, e.g. SID might be renamed to AdminSID, especially if the relationship was combined into Faqet. The old network data model would view FID FK -> FID PK as a relationship, which as described above and below is a very limited kind of relationship and not the approach taken by the ER model.
Another disadvantage of the network data model is predetermined access paths, which means we have to navigate from table to table using the predefined relationships. This complicated queries and data processing significantly. This limitation was one of the main drivers for the development of the relational model, to which the ER model maps. The understanding of tables as relations in the RM enables us to construct and navigate arbitrary access paths using joins. So, we do link tables in the RM, but at query time and as needed rather than at design time. The ER model is used for conceptual design only and doesn't describe relationships between tables, only relationships between entity sets.
Now, the ER model isn't a complete and consistent logical model like the RM, but it is a significant improvement over the network data model. An even more rigorous approach than ER would be object-role modeling, but that's a different topic.
I would like to create a model with two entities Orders and Products. They are linked by a relationship 'Contain' that has an attribute 'quantity'. How can I represent that in CoreData ? (Do not send me the ray tutorial, or any tutorial on youtube, I think I have done every thing). It is very important the relationship with attribute and not something general. (I know that it is not a database, but it is a Conceptual data model/Conceptual Schema as it is named by "entities" and "relationship" so if there is relationships, there must be a way to have relationships with attribute).
EDIT :
Am I doing the right thing by not adding id_order and id_product to the Contain entity ?
In CoreData, Contain would be another entity. It would have relationships to Orders and Products and a quantity attribute.
You cannot add attributes to a relationship in CoreData.
I'm using CoreData for the first time in one of my project. My table involves FOREIGN KEYS and since CoreData doesn't support FOREIGN KEYS, I'm having some issues.
Below is the structure of my tables.
My Problem is the establishment attribute.The establishment attribute is supposed to hold the name of a particular facility from the Facilities table. However, since it's a relationship, Xcode expects a Facility rather than just a name of a facility (NSString).
Is this possible, or am I just mixing up FOREIGN KEYS with RELATIONSHIPS in CoreData? How would I solve this problem?
Thanks in advance.
A relationship is not a property, so it does not have a type. In the model editor you add a relationship explicitly.
From your diagram, I see that you did not set the inverse relationship. There needs to be a corresponding relationship from name to the Assessors entity. (Set the "Destination" to Assessors in the model editor.)
I would also suggest to rename a few items.
First, use singular: Assessor, Facility. These are objects (comparable to classes), not tables.
Second, because your name attribute refers to an assessor, call it assessor, and similarly call its reverse relationship facilities (it is a to-many relationship in this direction, so the plural is appropriate).
If you need the name of the assessor of a facility, you use
facility.assessor.name
This should make it obvious why you do not need foreign keys. Indeed, I would urge you to think that it is not Core Data that does not support foreign keys, but that it is the traditional relational databases that do not support relationships!
I'm new to Core Data and I'm trying to implement it into my existing project. Here is my model:
Now, there's some things that don't make sense to me, likely because I haven't modelled it correctly.
CMAJournal is my top level object with an ordered set of CMAEntry objects and an ordered set of CMAUserDefine objects.
Here's my problem:
Each CMAUserDefine object has an ordered set of objects. For example, the "Baits" CMAUserDefine will have an ordered set of CMABait objects, the "Species" CMAUserDefine will have an ordered set of CMASpecies objects, etc.
Each CMAEntry object has attributes like baitUsed, fishSpecies, etc. that point to an object in the respective CMAUserDefine object. This is so if changes are made, each CMAEntry that references that object is also changed.
Now, from what I've read I should have inverses for each of my relationships. This doesn't make sense in my model. For example, I could have 5 CMAEntry objects whose baitUsed property points to the same CMABait object. Which CMAEntry does the CMABait's entry property point to if there are 5 CMAEntry objects that reference that CMABait? I don't think it should point to anything.
What I want is for all CMAUserDefine objects (i.e. all CMABait, CMASpecies, CMALocation, etc. objects) to be stored in the CMAJournal userDefines set, and have those objects be referenced in each CMAEntry.
I originally had this working great with NSArchiving, but the archive file size was MASSIVE. I mean, 18+ MB for 16 or so entries (which included about 20 images). And from what I've read, Core Data is something I should learn anyway.
So I'm wondering, is my model wrong? Did I take the wrong approach? Is there a more efficient way of using NSArchiver that will better fit my needs?
I hope that makes sense. Please let me know if I need to explain it better.
Thanks!
E: What lead me to this question is getting a bunch of "Dangling reference to an invalid object." = "" errors when trying to save.
A. Some Basics
Core Data needs a inverse relationship to model the relationship. To make a long story short:
In an object graph as modeled by Core Data a reference semantically points from the source object to a destination object. Therefore you use a single reference as CMASpecies's fishSpecies to model a to-one relationship and a collection as NSSet to model a to-many relationship. You do not care about the type of the inverse relationship. In many cases you do not have one at all.
In a relational data base relationships are modeled differently: If you have a 1:N (one-to-many) relationship the relationship is stored on the destination side. The reason for this is, that in a rDB every entity has a fixed size and therefore cannot reference a variable number of destinations. If you have a many-to-many relationship (N:M), a additional table is needed.
As you can see, in an object graph the types of relationships are to-one and to-many only depending on the source, while in rDB the types of relationships are one-to-one, one-to-many, many-to-many depending on both source and destination.
To select the right kind of rDB modeling Core Data wants to know the type of the inverse relationship.
Type Object graph Inverse | rDB
1:1 to-one id to-one id | source or destination attribute
1:N collection to-one id | destination attribute
N:M collection collection | additional table with two attributes
B. To your Q
In your case, if a CMAEntry object refers exactly one CMASpecies object, but a CMASpecies object can be referred by many CMAEntry objects, this simply means that the inverse relationship is a to-many relationship.
Yes, it is strange for a OOP developer to have such inverse relationships. For a SQL developer, it is the usual case. Developing an ORM (object relational mapper) this is one of the problems. (I know that, because I'm doing that for Objective-Cloud right now. But I did if different, more the OOP's point of view.) Every solution is a kind of unusual for one side. Somebody called ORM the "vietnam of software development".
To have a more simple example: Modeling a sports league you will find yourself having a entity Match with the properties homeTeam and guestTeam. You want to have an inverse relationship, no not homeMatches and guestMatches, but simply matches. This is obviously no inverse. Simply add inverse relationship, if Core Data wants and don't care about it.
I have a model graph, where there is several entities that have one to many relationship with other entity.
It means that inverse relationship should be one to many, but that is impossible.
The core data works well without inverse relationship; the only thing that drives me crazy is warnings:
Entity should have an inverse relationship
Does anybody know how to avoid these specific warnings?
Inverse Relationships
Most relationships are inherently bi-directional. If a Department has a to-many relationship to the Employees that work in a Department, there is an inverse relationship from an Employee to the Department. The major exception is a fetched property, which represents a weak one-way relationship—there is no relationship from the destination to the source (see “Fetched Properties”).
You should typically model relationships in both directions, and specify the inverse relationships appropriately. Core Data uses this information to ensure the consistency of the object graph if a change is made
Reference in Apple documentation
As a general rule, a one-to-many relation has a many-to-one inverse. In case you want to define a many-to-many relationship, you simply define one relation as to-many and its inverse as a to-many as well.