✔️ December 2022 moved all projects to .NET Core 5, EF Core 7
EF Core provides methods to transform one type into another. Most common use is with enumerations while many other transformations are possible. In this article/code sample there are two samples for enumerations and one for string array.
Bulk-configuring a value converter is possible as shown below.
Pre-defined conversions EF Core contains many pre-defined conversions that avoid the need to write conversion functions manually. Instead, EF Core will pick the conversion to use based on the property type in the model and the requested database provider type.
The code samples below allow a developer to get started by following along with provided code.
Code by itself is not enough, take time to read Microsoft documentation.
Check the following page for builtin converters.
🔸 Although there are many articles out there on transformations, this has ready to run examples.
🔸 Although I coded this repository, inspiration can from various incomplete post on the web.
public class CurrencyConverter : ValueConverter<Currency, decimal>
{
public CurrencyConverter()
: base(
v => v.Amount,
v => new Currency(v))
{
}
}In this example we want to categorize wines. The model has a primary key, string which represents the wine name and a enum for category,
public class Wine
{
public int WineId { get; set; }
public string Name { get; set; }
public WineVariantId WineVariantId { get; set; }
public WineVariant WineVariant { get; set; }
public override string ToString() => Name;
}Enum
public enum WineVariantId : int
{
Red = 0,
White = 1,
Rose = 2
}The following provides a one to many relationship
public class WineVariant
{
public WineVariantId WineVariantId { get; set; }
public string Name { get; set; }
public List<Wine> Wines { get; set; }
public override string ToString() => Name;
}Using HasConversion in the DbContext
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder
.Entity<Wine>()
.Property(e => e.WineVariantId)
.HasConversion<int>();
modelBuilder
.Entity<WineVariant>()
.Property(e => e.WineVariantId)
.HasConversion<int>();
modelBuilder
.Entity<WineVariant>().HasData(
Enum.GetValues(typeof(WineVariantId))
.Cast<WineVariantId>()
.Select(e => new WineVariant()
{
WineVariantId = e,
Name = e.ToString()
})
);
}Let's query for all wines
using System.Linq;
using HasConversion.Data;
using HasConversion.Models;
using Microsoft.EntityFrameworkCore;
using Spectre.Console;
using static HasConversion.Models.WineVariantId;
namespace HasConversion.Classes
{
public class WineOperations
{
/// <summary>
/// If database does not exists than pass true to this method
/// to create and populate with several records.
///
/// Otherwise passing false to view records in database
/// </summary>
/// <param name="reCreate"></param>
public static void AddViewWines(bool reCreate = false)
{
using var context = new WineContext();
var allWines = context.Wines.Include(item => item.WineVariant).ToList();
}
}
}Results
Display
To be consistent let's do the same pattern with books and category for books.
public partial class Book
{
public int BookId { get; set; }
public string Title { get; set; }
public BookCategory BookCategory { get; set; }
public override string ToString() => Title;
}
public class BookVariant
{
[Key]
public BookCategory BookCategoryId { get; set; }
public string Name { get; set; }
public List<Book> Books { get; set; }
public override string ToString() => Name;
}Setup the conversion, in this case we are not setting up for one to many but by following the pattern above we can.
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.HasAnnotation("Relational:Collation", "SQL_Latin1_General_CP1_CI_AS");
modelBuilder
.Entity<Book>()
.Property(e => e.BookCategory)
.HasConversion<int>();
modelBuilder
.Entity<BookVariant>().HasData(
Enum.GetValues(typeof(BookCategory))
.Cast<BookCategory>()
.Select(e => new BookVariant()
{
BookCategoryId = e,
Name = e.ToString()
})
);
}Get all books
var bookList = context.Book.ToList();Get by specific category Adventure
var list = bookList.Where(books => books.BookCategory == BookCategory.Adventure).ToList();
When working with enum, consider placing the enum values in a database table and use a T4 template to generate the model. This will be helpful when there are many projects using the same enum and saves time if a member name changes, members are deleted or added.
🔸 There are several examples included.
This one although provides a decent sample for using HasConversion the data model can be improved. This comes from the following forum question.
I know I can create classes from the JSON data, and parse out the elements. Butttt how do I then insert into MS SQL Server tables?
Their code
public class Account
{
public string Email { get; set; }
public bool Active { get; set; }
public DateTime CreatedDate { get; set; }
public IList<string> Roles { get; set; }
}
string json = @"{
'Email': '[email protected]',
'Active': true,
'CreatedDate': '2013-01-20T00:00:00Z',
'Roles': [
'User',
'Admin'
]
}";
Account account = JsonConvert.DeserializeObject<Account>(json);
Console.WriteLine(account.Email);One forum member recommended to string string concatenation which is okay but EF Core can handle this.
My recommendation
Model which changes the Roles property to a string array.
public partial class Account
{
public int Id { get; set; }
public string UserName { get; set; }
public string Email { get; set; }
public bool? Active { get; set; }
public DateTime? CreatedDate { get; set; }
public string[] Roles { get; set; }
}Conversion
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.HasAnnotation("Relational:Collation", "SQL_Latin1_General_CP1_CI_AS");
modelBuilder.Entity<Account>()
.Property(e => e.Roles)
.HasConversion(
value => string.Join(',', value),
value => value.Split(',',
StringSplitOptions.RemoveEmptyEntries));
OnModelCreatingPartial(modelBuilder);
}
View records
public static void ViewAccounts()
{
using var context = new AccountContext();
var accountList = context.Account.ToList();
var table = CreateViewTable();
foreach (var account in accountList)
{
if (account.Id.IsEven())
{
table.AddRow($"[bold yellow on green]{account.Id}[/]", $"[bold yellow on green]{account.UserName}[/]");
}
else
{
table.AddRow($"{account.Id}", account.UserName);
}
foreach (var role in account.Roles)
{
table.AddRow("", role.PadLeft(10));
}
table.AddEmptyRow();
}
AnsiConsole.Write(table);
var admins = accountList.Where(account => account.Roles.Contains("Admin")).ToList();
}In this case a List<int>
Model
public class EntityType
{
public int Id { get; set; }
public List<int> ListProperty { get; set; }
}Conversion
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder
.Entity<EntityType>()
.Property(e => e.ListProperty)
.HasConversion(
v => JsonSerializer.Serialize(v, null),
v => JsonSerializer.Deserialize<List<int>>(v, null),
new ValueComparer<List<int>>(
(list1, list2) => list1.SequenceEqual(list2),
c => c.Aggregate(0, (a, v) => HashCode.Combine(a, v.GetHashCode())),
c => c.ToList()));
}Add record
var entity = new EntityType { ListProperty = new List<int> { 1, 2, 3 } };
context.Add(entity);
context.SaveChanges();Result
This converter transforms a bool to string where the value in the database table will be a string then when read back as a bool.
public class Person
{
[Key]
public int Id { get; set; }
public string FirstName { get; set; }
public string LastName { get; set; }
public bool IsFriend { get; set; }
public override string ToString() => $"{FirstName} {LastName}";
}Taken from a Stackoverflow post.
Is there a way to indicate that this is not a relationship but should be stored as a big string?
public class Campaign
{
private string _extendedData;
[Key]
public Guid Id { get; set; }
[Required]
[MaxLength(50)]
public string Name { get; set; }
[NotMapped]
public JObject ExtendedData
{
get
{
return JsonConvert.DeserializeObject<JObject>(string.IsNullOrEmpty(_extendedData) ? "{}" : _extendedData);
}
set
{
_extendedData = value.ToString();
}
}
}In the DbContext
protected override void OnModelCreating(ModelBuilder modelBuilder)
{
modelBuilder.Entity<Campaign>()
.Property<string>("ExtendedDataStr")
.HasField("_extendedData");
}In this article there are enough code to run and study to learn the basics of transforming properties in models using Entity Framework Core. Couple this with Microsoft docs and the links below a developer can easily perform conversions.
🔸 Some coders may look for places to use what has been presented while the reverse should be the path, tuck these away and when a situation arises you have a solution.
Before EF Core 6.0, the generic overloads of the HasConversion methods used the generic parameter to specify the type to convert to.
- khalidabuhakmeh Entity Framework Core 5 Value Converters
- Entity Framework Core – Improved Value Conversion Support
- EF Core 7 code sample for Json Columns
Spectre.Console provided classes to provide clear ways to present data. Package.
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