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Magento – Storing product attributes of different types in a product aggregate

I use the EAV (Entity-Attribute-Value) pattern in an inventory management system I’m developing.
It’s very comparable to a typical webshop framework like Magento.

Attributes have a data type, like numeric, varchar, text.
The attribute definitions are stored in one table like eav_attributes. The values are stored in separate tables specific for the data type.
Say that a numeric attribute is assigned to a product. The database table that holds the relation between the attribute and product and the value, is a table like eav_decimal_attribute_values.
Example table scheme:

attribute_id | product_id | attribute_value (decimal)

And the same principle for a varchar attribute:

attribute_id | product_id | attribute_value (varchar(255))

When loading a product from the database, I load the product -> attribute -> value relations of each of that tables.
So I query the eav_decimal_attribute_values table to get all assigned decimal attributes and their values for the product (or get an empty resultset or course).
And the same for the other attribute types (eav_varchar_attribute_values, eav_text_attribute_values, …).
So far this is actually the same as how e.g. Magento manages this.

Since I use a typed language, I chose to have the following design:

class AttributeDefinition
    - string attributeName
    - string attributeDescription
    - enum dataType // Like 'numeric', 'varchar', 'text', ...
    - ...
    - ...

abstract class ProductAttributeAssignment
    - AttributeDefinition attribute

class DecimalProductAttribute : ProductAttributeAssignment
    - double value
    - ...

class VarcharProductAttribute : ProductAttributeAssignment
    - string value
    - ...

class TextProductAttribute : ProductAttributeAssignment
    - string value
    - ...

Product
    - name
    - ...
    - attributes
        // Problematic part: storing different types together
        - A TextProductAttribute 
        - Another TextProductAttribute
        - A DecimalProductAttribute
        - A VarcharProductAttribute
        - ...

In code I want to preserve types. Like a numeric attribute’s value is stored as a double or decimal value in code. A varchar or text attribute is put in a string.
Of course I could just create one concrete class ProductAttributeAssignment, and store the value always as a string in memory, regardless of the data type the attribute represents. But imagine that I have to do calculations with numeric attributes, or search features like Charging cable length > 120, it would be useful to preserve types. Also when there would be features like developers could create plugins or hooks.

The problem is that I have some difficulties in coding the product aggregate. It would be logical to store a products’s attribute assignments just in a Product object. But since there’s a different class for each attribute type, I cannot just write a class like Product with a field List<…> attributes.

So, my question is not about storing things in the database/persistent layer, but about writing code for a business object that owns things that are actually the "same business thing" but technically different types in code.

Tags:

2

Answers


  1. 0

    I didn’t pick up on any database libraries or frameworks or anything in your post, so I’m assuming you’re just doing this in pure C#. You could do this with a list containing generic classes, (inspired by this post), either by keeping a List<object>, or a common parent interface/class.

    public class Product {
  // ...
  public List<AttributeBase> Attributes { get; private set; } = new List<AttributeBase>();
  // ...
}

public abstract class AttributeBase {
  public AttributeDefinition Definition { get; private set; }
  public abstract object ObjectValue { get; }

  public AttributeBase(AttributeDefinition definition) {
    Definition = definition;
  }
}

public class Attribute<T> : AttributeBase {
  public object ObjectValue => Value;
  public T Value { get; set; }
  
  public Attribute(AttributeDefinition definition, T value) : base(definition) {
    Value = value;
  }
}

    And then you could use it like…

    Product product = ...;

// Adding an attribute...
AttributeDefinition definition = ...;
Attribute<double> newAttribute = new(definition, 1.23);
product.Attributes.Add(newAttribute);

// Getting an attribute...
AttributeBase attribute = product.Attributes[0];

// You can get the value like this...
object valueAsObject = attribute.ObjectValue;
if (valueAsObject is double castedDouble) {
  Console.WriteLine(castedDouble + 0.654);
}

// OR like this...
if (attribute is Attribute<double> doubleAttribute) {
  double value = doubleAttribute.Value;
  Console.WriteLine(value + 0.654);
}

// Example, Finding the attribute named "ProductSKU"
AttributeBase skuAttribute = product.Attributes.FirstOrDefault(attrib => attrib.Definition.name == "ProductSKU");

// Since WriteLine can take an object, it doesn't matter.
Console.WriteLine(skuAttribute.ObjectValue);

    Or, if you wanted to avoid generics, you could just make child classes for each type, which allows for additional parameters and validators like a varchar’s size. (you could even mix both…):

    public class VarCharAttribute : AttributeBase {
  public object ObjectValue => Value;
  public string Value { 
    get => _value; 
    set {
      if (value.Length >= MaxLength) {
        value = value.Substring(0,MaxLength);
      }
      _value = value;
    }
  }
  public int MaxLength { get; private set; }

  private string _value;

  public VarCharAttribute(AttributeDefinition definition, string value, int limit) : base(definition) {
    MaxLength = limit;
    Value = value;
  }
}

    But even something like this could have been done with the generic attribute using a class. There’s tons of ways to do this, my solution is just one of them; you have to pick whichever one would work best for your particular use-case.

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  2. 0

    I think what @ipodtouch0218 suggested..
    You can use a generic collection -> AttributeDefinition,
    Based on the type double/string/float it will save the value and type would be retained for you.
    Keeping it generic would be fine considering EAV pattern.

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