Dynamic view models - oop

If I'm about to split my application into read and write sides using CQRS principles how would you handle situation of having a "dynamic" read model scenario? E.g. I have a product that consists of several read properties (id, title, slug etc) but in order to display it to user I need to pass its price that is calculated by domain service (to which I pass visitor country and currency)? I can't store this price in database because user is free to change his currency anytime he wants + prices change quite often. So my final product view model should be basically a composite of raw-SQL columns (id, title, slug) and calculated price. Should I use the same view model that I get from the database and use content enrichment pattern or should I create a new view model for my composite?

Changing the price of the product is a COMMAND, it should update the price you store in the database which you then QUERY anytime.
The essence of CQRS is that your queries are not going any logic like that.
For currency conversion, since you say the user can change currency at anytime then you should query the entire list of currency rates and do the calculation yourself on the client side. Alternatively you could query for the base price then query for the current rate. And if the user changes currency you just query for the rate again but keep the same base price.

Related

Database design for a product-configurator

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.

DDD - Entity vs ValueObject

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!

Where are the calculated fields from Odoo stored in the sql server when stored is set to true

If I create a new field on a model, making it a calculated or related field, and set the store parameter to be true. When I look on the sql server using pg admin, on the table itself, i can't fiend the field. I think this would be logic if the data entered wouldn't be stored, but calculated each time it was shown on a view (from, tree, ...).
Example in the POS orderline the field tax_ids is a calculated / related field. The tax_id is stored, because if I change the id on product level, it doesn't affect the order line, when consulting it after changing the tax id. So this data is stored. But when I try to find the field in the pos_order_line model on the sql server it doesn't show on the pos orderline. In odoo, settings models, it does show on the pos_order_line model. So where does odoo store the data?

Prestashop: use BlockLayered class methods and/or DB tables for price range button?

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.

WCF Service call to multiple rows. How to architecture it?

Let's say I have a scenario where I have a Service that is responsible for retrieving the price of a single product given the ID and another Service that gives me the current stock position of it.
It's okay when I'm looking only at one product, but what about a list page where I have 60 products? What should I do in this case? Call the service product by product to retrieve it's current price and stock position?
I think this would be extremely slow and cost a lot of resources. But what could I do to make it look good and at the same time have performance?
Currently we have these information in the database in a column next to the product. These price and stock columns are updated by a sql service that updates it when is necessary so when I need to grab the value for a lot of products at the same time I just need to select two columns more. It's really fast, but right now we have some more systems in need of this information and I would like to transform all these stuff in a service.
Thanks a lot!
Well, you could always have multiple service methods:
the one you already have, to retrieve a single product, and the price for a single product
create a new service method which would take a list of product ID's, and return a list of product objects - or a new DTO (data-transfer object) that holds product and price
This way, you could keep your current users happy, and also do something to make batch requests work more efficiently.
Can your service method take an array of IDs? And return an array of values? That way if you want one record your array only has 1 item, if you want more, you just populate the array with multiple values?