Working in Spring, and using H2 for now.
So, I have these two classes/Entities with their repositories working fine if I keep it simple.
Brand, which simply has a name and a price;
and VendingMachine, which has a model and some other properties;
BUT I need to implement this:
Different VendingMachines sell different Brands, for which they have a stock. The Brands it sells are determined when you instantiate the class.
So my idea is that the table for VendingMachines fields should look something like that:
MODEL////ID/////MAXCAPACITY////MAXBRANDS///BRAND01////BRAND02///BRAND03...
Where all the BRAND columns are the different Brands the machine sells, assigned at its instantiation as said before. The value will be an Integer.
My original idea was to send this data to the #Entity via a HashMap(Brand, Integer) but I don't know how to make it work once it tries to fit it into the JpaRepository, or even if it is possible this way.
Probably missing some magic Annotation I don't know.
Thanks, hope my question was well explained for everyone.
Extract Brand as new table/entity(read about database normalization).
And after just do one-to-many relationship with that entities(VendingMachines and Brand). would look like that:
class Brand
...
#ManyToOne
#JoinColumn(name="machine_id")
private VendingMachines machines;
Related
I have been asked by a customer to develop a "product configurator", and i need some inputs on how to handle the DB part of the project.
Each product can have a subset of different precreated attributes.
The minimum is 1 attribute, but there is no maximum.
Some attributes have dependencies/relationships with other attributes.
Eg. If the product is a chair, you need to choose the material (wood, plastic, metal), and you need to choose which type of legs the chair shoud have.
If the Product is a cabinet, you still need to choose a material, but instead of legs there will be different doors to choose from etc.
Each of these attributes might have subattributes. Eg. the door has a color, a size and a doorhandle.
Then the door handle has a material, a type and so on.
This ultimatly ends up in a multi-layered attribute-tree.
By itself this isnt too complicated to code, however the customer wants to be able to manage (Create, update and delete) all products, attributes and relationships between attributes, within the webapp.
So coding the relationship-part isn't a viable solution.
I have gone with a EAV model to facilitate the "potential unlimited" amount of attributes each product can have.
But i am struggling to figure out how to go about the "attribute relationships".
A simplified version of my DB design looks like this:
If each product could subscribe to groups of attributes that is legal. Then each attribute belongs to a group like "wood group".
Then the user could set the groups of attributes against a product that should need to be answered to configure a product.
With regards managing a tree, you could use a column type of hierarchyid . Or construct an outline string as key field.
An outline for example
1.
1.1.
1.1.1.
1.2.
2.
2.1.
I was reading about DDD and I realize that sometimes an entity might be a VO or VO might be an entity. You can know which one is better depends on the context. I was checking different examples. For example, shopping cart DDD example. Product is an aggregate root, Cart is an aggregate root and items is a entity of Cart, so if you want to add a product to the cart you would do something like this:
$cart->addProduct(id $id, $name, $price)
class Cart
{
private items;
function addProduct(ProductId $id, ProductName $name, ProductPrice $price) {
this->items[] = new Item(
new ItemProductId($id->ToString()),
new ItemName($name->ToString()),
new ItemPrice($price->ToString()),
new ItemCartId(this->id->ToString())
);
}
}
There are two reasons why I think it is a VO:
You cannot modify the value's item ( only if the product's
price has been modify there is a event that would modify its price).
The item doesn't have id, it's has a reference of the
product(ItemProductId) and a reference of the cart (ItemCartId)
I was reading about DDD and I realize that sometimes an entity might be a VO or VO might be an entity. You can know which one is better depends on the context.
Usually its pretty clear whats entity and whats an value object. If it contains data that's fixed at the time of assignation, its a value object. For example "order address" on the order aggregate. When the order is placed, the address is set. "Addresses" may be an entity in user aggregate (i.e. a list of his common addresses), but for an order its an value object since its not supposed to change when the user edits or deletes one of his addresses.
cart->addProduct(id $id, $name, $price)
class Cart
{
private items;
function addProduct(ProductId $id, ProductName $name, ProductPrice $price) {
this->items[] = new Item(
new ItemProductId($id->ToString()),
new ItemName($name->ToString()),
new ItemPrice($price->ToString()),
new ItemCartId(this->id->ToString())
);
}
}
That's a pretty bad example. Why would or should the value object be ItemPrice? Does that makes it any special? Why string? A price is usually just a numeric value but also involves a currency, passing it as string kinda beats that.
On top of that, having ItemCartId in the it does
a) leak data persistence knowledge into your domain. The fact, it's contained inside this->items[] already establishes a relationship between the entity (or aggregate) and the value object. ItemCartId as no meaning in the domain, other than that it's required for relational database engines (=persistence knowledge)
There are two reasons why I think it is a VO:
You cannot modify the value's item ( only if the product's price has been modify there is a event that would modify its price).
You sure? Why would a eCommerce business want to have the prices in the card anyways?
Prices are informational only, they could change before the order is placed. Same as availability.
A lot of users put stuff in their cart and check on next day. In that time, the price could change.
No company would want to sell a product for the price when it was put into the shopping cart, if the price increased in the time since it was put in there. That would mean a financial loss.
Prices in the shopping carts are informational, not compulsory. You need know the exact process of the company.
The item doesn't have id, it's has a reference of the product(ItemProductId) and a reference of the cart (ItemCartId)
Again. Why do you think ItemCartId belongs to the Item object? That's leaked persistence knowledge, since its only important for relational database systems.
All you really need in a shopping cart is
* product or article number (not necessary the id, that's typically db knowledge)
* quantity
Nothing else. If you may want to change the user when the price changed and show the old and new price, the take the price (=currency value object, not ItemPrice) to it too as a value to compare to an old state.
Finally and probably most importantly: Consider if the shopping cart is an aggregate at all (or does fit into ddd).
After all, most shopping carts are only a value bag w/o a lot of business logic into it. The real logic (checking the real price, product availability, asking for shipping location, calculation of taxes and shipping costs) happens during the checkout process, not while putting stuff into the cart.
For example you can check out eShops on Containers demo project showing an example shopping service based on microservices and ddd.
Some microservices apply DDD (such as Ordering microservice), where others don't (Catalog microservice or the Basket (cart) Microservice).
Applying DDD doesn't mean everything needs to be done with DDD. If its simple crud based services, then you don't need DDD for these. DDD adds a value where you have complex systems and complex business logic.
A catalog has neither, it just presents data which come from a different system (i.e. ERP which on other side may be built on using DDD).
I don't understand what are you asking exactly, but the code you are providing could be improved.
First of all I suggest you to read the red book by Vaughn Vernon https://www.amazon.co.uk/Implementing-Domain-Driven-Design-Vaughn-Vernon/dp/0321834577: you can find 3 chapters describing how to define entities, value objects and aggregates, with some rules of thumbs.
One of those advices, is to keep your aggregates as small as possible, in order to improve your performance and keep the code easy to read and maintain. Imagine that you have a Blog aggregate that contains a list of Post entities: if you manage all of them in a single aggregate, when you want to modify the blog Author, for example, you are forced to retrieve all of the blog's post, without any reason, and that means that you are doing a join and slowing down your application. The more your aggregates grows, the slower those queries with their joins.
So, in the case of the Cart, I suggest you to build the cart without any item or product, instead you can add the CartId to the Item. Cart does not know which items it contains, but items know in which cart they are.
About value objects: is a tool that allows you to wrap some validation and business logic inside a class that is represented by its state and not by its id (like entities), so in the case of the cart, if you put two identical bottles of water inside it, how can you know that they are different? Do you need to know that they are different? Are they different if they are physically (or logically different) or are they different if some of their attribute is different?
In my opinion an item or a product, in your case, are entities because they are not measuring anything, and when you put an item twice, you actually have two different items (and you use an id to recognize them).
This is not necessary like this, sometime you can use a value object and sometimes an entity, it depends on your context. A good example to understand that difference is with money:
if you want to measure an amount, for example 10 dollars, probably a value object will work for you, because you don't care if it a bill or another, you just want to measure 10 dollars; in this case if you have 10 dollars, is not important if you have a bill or another, the important thing is that is 10 and not 5
in the case that you need to recognize different physical bills, for any reason (you need to track money for the police), you should use an entity, because any printed bill has a unique serial number, and a 10 dollar bill, in this context, is actually different from another 10 dollar bill
Hope this can help you.
Goodbye!
I'm working on a proof of concept to explore how to connect SPARQL data from different data sources. I have two OWL classes, each from its own data model, representing information that I've extracted from two different systems. The data models look like:
Order: id, itemName, quantity
Item: id, name, unitPrice
What I would like to do is connect these two types together using the Order.itemName and Item.name fields, and create a new field on the Order class to store the total price for the order (order.quantity * item.unitPrice).
This would be pretty trivial to do with SPARQL, but we're looking at TopBraid Composer due to its graphical mapping capabilities. From my understanding, using a SPINMap we should be able to easily - and graphically - define the above property, but I'm having some problems getting things connected together.
I've been following the tutorial in the TBC Help under "Application Development Tools/SPIN/Ontology Mapping with SPINMap", but I'm having problems just trying to get a simple attribute mapped over. Based on that help document, on the Diagram tab I'm drawing a line from the Item Class to the Order class and selecting the changeNamespace function with an appropriate value. I then drew a line from the Item class to a new property on the Order class, totalPrice, and set the function to equals, just to see if I can get the property to show up. After running the inferences, I run a SPARQL query, and the totalPrice field is empty for all of my Order classes. I know this description was confusing - any idea what I should be doing to make this work?
Matt, in your second step are you using a property from the Item class to map to the Order class? That should work without any problems for the 'equals' template.
Check the Inferences tab in the middle-bottom row of tabs aster running inferences. This will show all materialized inferences and can be used to verift hat your SPINMap is doing what is desired.
Also look at Configure Inferencing to make sure the TopSPIN reasoner is chosen.
Good afternoon everyone!
I'm studying NHibernate, and decided to make some changes. Among them, I noticed that some fields are unnecessary. So I bring my doubt:
I have a list, let's call it Class_List within each study class, I can have N students for each class. Within the list Class_List, I also have other properties as simple as the name of the class.
How I see it is unnecessary to store how many students I have in the database, I would, in a single query, how many records I have. This, using NHibernate.
Is this possible? How?
Best regards,
Gustavo.
Edit: I've forgot to say one thing... I want to return this number of record, as a column. But this column is not mapped in my .hbm.xml file.
If students are mapped as a collection on Class, you can try using something like this:
var numberOfStudents = session.CreateCriteria<Class>()
.Add(Restrictions.IdEq(1))
.CreateCriteria("_students", "students")
.SetProjection(Projections.RowCount())
.UniqueResult<Int32>();
Where '1' is the id of the class (you can use other property) and '_students' is the name of the students collection.
I have a problem naming the elements in my application's data model.
In the application, the user has the possibility to create his own metamodel. He does so by creating entity types and a type defines which properties an entity has. However, there are three kinds of entity types:
There is always exactly one instance of the type.
For instance, I want to model the company I am working for. It has a name, a share price and a number of employees. These values change over time, but there is always exactly one company.
There are different instances of the type, each is unique.
Example: Cities. A city has a name and a population count, there are different cities and each city exists exactly once.
Each instance of the type defines multiple entities.
Example: Cars. A car has a color and a manufacturer. But there is not only one red mercedes. And even though they are similar, red mercedes #1 is different from red mercedes #2.
So lets say you are a user of this tool and you understood the concept of these three flavors. You want to create a new entity type and are prompted to choose between option 1, 2 and 3. How would you name these options?
Edit:
Documentation and help is available to the user. Also the user can be expecteted to have a technical/programming background, so understanding these three concepts should be no problem.
First of all let me make sure I understand the problem,
Here's what you have (correct me if I'm wrong):
#of instances , is/are Unique
(1,true)
(n,true)
(n,false)
If so,
for #of instances I would use single \ plural
for is\are unique (\ not unique) I would use unique \ ununique.
so you'll get:
singleUnique
pluralUnique
pluralUnunique
That's the best I could think of.. I don't know exactly who are your users and what is the environment, But if you have an option of adding tips (or documentation) that should be used for sure.