Flattening of complex (object) properties of an entity

[satano]

This is detailed info about Support for complex entities, whose properties are another objects from issue #8.

The goal

We want to load (and save) from database entities with complex properties. Complex property is a property which is another object. In its simplest form, say we have this entity Invoice:

public class Invoice
{
    int Id { get; set; }
    string InvoiceNumber { get; set; }
    Address Supplier{ get; } = new Address();
    Address Purchaser { get; } = new Address();
}

public class Address
{
    public string Street { get; set; }
    public string City { get; set; }
    public string Country{ get; set; }
}

And we want to automatically map it to table with these columns:

• Id
• InvoiceNumber
• SupplierStreet
• SupplierCity
• SupplierCountry
• PurchaserStreet
• PurchaserCity
• PurchaserCountry

What/how should work

• Complex properties with injector set must not be flattened.
• Column name will be defined as concatenated names of properties:
  Supplier.Street -> SupplierStreet
• Deep flattening of properties:
  Invoice.Supplier.Address.Street -> SupplierAddressStreet
• Setting specific column name for nested property:
  .Property(p => p.Supplier.Address.Street).HasColumnName("SupplierStreet")
• Complex properties can be readonly ({ get; }) or read/write ({ get; set; }).
  • If the property is null, instance should be created automatically if it is possible (property is r/w and property's type has parameterless constructor).
• Can set nested properties as not mapped:
  .Property(p => p.Supplier.Country).NoMap()
• Can use injectors for nested properties:
  .Property(p => p.Supplier.Country).InjectValue(...)
• Update readme.

Other considerations

• Should flattening work automatically and be turned off if not wanted, or vice versa?
• We will support only fluent configurations for this (no attributes).
• Can we make whole attribute configuration obsolete?