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 want to do a autoshop software... where they keep up the cars they have and what they need(engine and other parts for example) but i dont know how to do the database to accept multiple items at once
example:
a car needs on one visit to the auto shop:
left frontal door
tires
oil change
filters
how to i add this in one go to the database(with prices included) so that i can see it all after and print a bill wheer it shows all... but my main priority is being able to insert all in one go and in one table
Hard to tell without any idea about your db strucutre. Lets assume db isn't constructed yet, you don't want to decrease parts stock or keep any track of wich exact part (i mean with serial etc.) was used. You want it quite simple, just a table with a car bought some parts.
In this case i woulde use a table looking like this : id|date|car_id|parts_used
where parts_used is a string containing parts and prices with separators. For example : "left frontal door=500+tires=100+oil=10" and then split the string when reading db.
I'm not sure it's what you want but your question isn't quite precise :)
I've got to build a standalone menu button with submenu that contains links to price ranges.
I activated the blocklayered module (not for this task, only for regular left-column filters). So the relative db tables are in place and populated.
I want to make a controller specific for price ranges. So I've got to do the right query and maybe set up the same url vars as the blocklayered module so they wil not conflict.
Would it be too crazy to import blocklayered or blocklayered-ajax in my controller and use part of their functionality? Maybe not good because of object duplication or other issues?
Or maybe, would it be a bad idea to use the blocklayered tables (for example layered_price_index) to help me get filtered products? I'm wandering if it would be a better solution than re-doing all by myself, or if instead it's not good for some reason.
Any idea?
It really depends on which amount (among the ones below) you would like to take into account in your price range filter:
Amount without taxes
Amount including taxes
Amount including discount/promotion
Amount in several currencies or only one currency
Amount for a specific customer group or for everyone
Amount base on any other product price rule
The easy way:
You can build a price range controller easily by yourself, handling only a single currency and prices without taxes and reduction. It will probably be 90% accurate (because of the missing discounts a product might not show up for a certain range).
In that case, you can easily build a query on the ps_product and ps_specific_price tables and SELECT in real-time the right products for a given range.
The proper way:
You want to handle discounts, price rules, specific prices, etc. If you build a real-time query including all these calculations and parameters, it may slow down the server.
Build a product price cache or re-use the one setup by the Block Layered module.
I'm working on using a recipe database in SQLw, like the one in this question (which has helped a lot already) Structuring a recipe database , to combine the ingredients of several user selected recipes to a shopping list.
Also, the items on this shopping list are to be divided in two categories (eg: "groceries" and "check pantry")
Example case:
User can select 7 recipes to make a weekly mealplan in a form (almost got this part)
The given output is a shopping list of all the ingredients marked as "groceries" and a "check stock" list of all the ingredients marked as "pantry".
Any help at all would be much appreciated!
I had just posted a full solution, but given the subject here looks like it may be homework, I'm just going to point you in the right direction. If this isn't homework, leave a comment and I'll put the full solution back.
Since you have multiple recipes, a normal selection based on joins would give you back multiple rows per ingredient. You want some way to roll up all of the rows for a given ingredient into a single row and show a total of the quantity that you need.
I have a table that has ordernumber, cancelled date and reason.
Reason field is varchar(255) field and it was written by many different sales rep and really hard to group by the reason category I need to generate a report to categorize cancelation reasons. What is the best way to analyse the reasons with TSQL?
Sample of reasons entered by sales rep
cust already has this order going out
cust can not hold for item Called to cancel order
cust doesn't want to pay for shipping
wife ordered same item from different vendor, sent email
cst made a duplicate order, sent email
cst can't hold
Cust doesn't want to go through verification process so is cancelling order
doesn't ant to hold for Bo
doesn't want
Cust called to cancel the order He can no longer get the product he wants
cnt hld
will not comply with export req
cant' hold
Custs request
Cust will not hold for BO
per. cust. request.
BTW I have SQL Server 2005.
part of your problem is that this these aren't truly reason codes. sounds like an issue with your schema to me. if there aren't predefined reason codes to reference and you're allowing free text entry for each reason, then there's really no way to do this directly, outside of pulling distinct reasons back, which is probably not going to be very useful.
just an idea, can you add another column to the table, even if it's in a temp or test environment and then give the business users the ability to assign a code (e.g. 1 for mis-ships, 2 for duplicate orders, 3 for wrong item etc.) to each order cancellation. then perform the analysis on that.
i assume that's what they're expecting from you, but i don't know that i see any better way. you could always perform the analysis yourself if you have the authority/knowledge but this might be painful if you have a ton of cancellations.
edit- i see now that you've tagged this with regex... it would be possible to setup specified keywords to pull out the entries, but there'd have to be some tolerance built in and still manual analysis afterwards for items which don't fall into any specified category due to misspellings etc. /edit
+1 to #jmatthews, you really need to have reason codes that are selected and then possibly allow free-form entry for the full reason.
If this isn't an option you can look into text clustering. Don't expect that to be fast or easy though, it's still an open research topic and is related to both AI and machine learning.
Look at Term Lookup in SSIS, here is an article to read.