Mapping a long string ?

[ignatandrei]

I do like the library . However, one question : If I want to map a long string, do you have a suggestion ?

[dualbios]

Hi @ignatandrei , thanks. Could you please give an example of the data structure and data that you want to map?

[ignatandrei]

  {
    "Id": 2,
    "Stage":"done",
    "Title": "NullObject",
    "Description":"Instead of returning null , use an object which implements the expected interface, but whose method body is empty.",
    "LinkWikipedia":"https://en.wikipedia.org/wiki/Null_object_pattern",
    "DemoFileCsproj":"NullObject.csproj",
    "ClassNames":"EmptyFolder,NullLogger",
    "Homework":"When retrieving data( e.g. a Person with ID =-1 ) from a database , return a NullObject instead of null. How you will verify that the object is a NullObject?",
    "Tags":"behavioral,design pattern"    

  }

I do with description and Homework to be as an array ( each item of the array could be a line )

[ignatandrei]

Any suggestions?

[dualbios]

Hi, thanks for example. Working on it.

[dualbios]

Hi @ignatandrei as I understand you want to set the type of 'Homework' to 'string[]', like that

{
                    "Id": 2,
                    "Stage": "done",
                    "Title": "NullObject",
                    "Description": "Instead of returning null, use an object which implements the expected interface, but whose method body is empty.",
                    "LinkWikipedia": "https://en.wikipedia.org/wiki/Null_object_pattern",
                    "DemoFileCsproj": "NullObject.csproj",
                    "ClassNames": "EmptyFolder,NullLogger",
                    "Homework": [
                        "When retrieving data (e.g. a Person with ID =-1) from a database, return a NullObject instead of null.",
                        "How will you verify that the object is a NullObject?"
                    ],
                    "Tags": "behavioral,design pattern"
                }

Unfortunately, the EF does not support an array and throws an exception 'The property 'DesignPattern.Homework' could not be mapped because it is of type 'string[]', which is not a supported primitive type or a valid entity type. Either explicitly map this property, or ignore it using the '[NotMapped]' attribute or by using 'EntityTypeBuilder.Ignore' in 'OnModelCreating' when trying it.

If you have comments on it you can try to play with it in the 'try-to-support-string-array' branch and let me know.

[dualbios]

In case when you need to have 'Homework' property as a 'string[]' you can use a converter for that property. https://learn.microsoft.com/en-us/ef/core/modeling/value-conversions?tabs=data-annotations

[ignatandrei]

How this value converter could be coded, since, you said previously, "could not be mapped because it is of type 'string[]', which is not a supported primitive type or a valid entity type" ?

[dualbios]

Creating a value converter in Entity Framework Core (EF Core) to convert a string[] to a string and vice versa involves implementing a custom ValueConverter. This converter will be used to serialize and deserialize the string[] to a string when storing in the database. Here's how you can create such a converter:

Step-by-Step Guide

1. Create the Value Converter Class:

First, create a class that inherits from ValueConverter<string[], string> and implements the conversion logic.

using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
using System;
using System.Linq;

public class StringArrayToStringConverter : ValueConverter<string[], string>
{
    public StringArrayToStringConverter()
        : base(
            v => ConvertToString(v),
            v => ConvertToArray(v))
    {
    }

    private static string ConvertToString(string[] array)
    {
        if (array == null || array.Length == 0)
        {
            return null;
        }
        return string.Join(",", array);
    }

    private static string[] ConvertToArray(string str)
    {
        if (string.IsNullOrEmpty(str))
        {
            return Array.Empty<string>();
        }
        return str.Split(',', StringSplitOptions.RemoveEmptyEntries);
    }
}

2. Configure the Converter in Your DbContext:

In your DbContext class, you need to apply the value converter to the relevant property using the ModelBuilder.

using Microsoft.EntityFrameworkCore;

public class YourDbContext : DbContext
{
    public DbSet<YourEntity> YourEntities { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        var converter = new StringArrayToStringConverter();

        modelBuilder.Entity<YourEntity>()
            .Property(e => e.YourStringArrayProperty)
            .HasConversion(converter);
    }
}

3. Define Your Entity:

Ensure your entity has a property of type string[].

public class YourEntity
{
    public int Id { get; set; }
    public string[] YourStringArrayProperty { get; set; }
}

Example Usage

Here is a complete example with a simple DbContext, an entity, and the custom value converter:

1. Entity:

public class YourEntity
{
    public int Id { get; set; }
    public string[] Tags { get; set; }
}

2. DbContext:

using Microsoft.EntityFrameworkCore;

public class YourDbContext : DbContext
{
    public DbSet<YourEntity> YourEntities { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

        var stringArrayConverter = new StringArrayToStringConverter();

        modelBuilder.Entity<YourEntity>()
            .Property(e => e.Tags)
            .HasConversion(stringArrayConverter);
    }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        // Configure your database connection here
        optionsBuilder.UseSqlServer("YourConnectionString");
    }
}

3. Value Converter:

using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
using System;
using System.Linq;

public class StringArrayToStringConverter : ValueConverter<string[], string>
{
    public StringArrayToStringConverter()
        : base(
            v => ConvertToString(v),
            v => ConvertToArray(v))
    {
    }

    private static string ConvertToString(string[] array)
    {
        if (array == null || array.Length == 0)
        {
            return null;
        }
        return string.Join(",", array);
    }

    private static string[] ConvertToArray(string str)
    {
        if (string.IsNullOrEmpty(str))
        {
            return Array.Empty<string>();
        }
        return str.Split(',', StringSplitOptions.RemoveEmptyEntries);
    }
}

Summary

This approach uses EF Core's ValueConverter to handle the conversion between string[] and string. When saving the string[] to the database, it will be serialized into a single comma-separated string. When retrieving it, it will be deserialized back into a string[]. This is particularly useful when you want to store arrays in a single database column while still working with them as arrays in your application code.

[ignatandrei]

The problem is " When saving the string[] to the database, it will be serialized into a single comma-separated string"

This is something that is difficult in JSON . So instead of having

{ "description":"One long sentence . And another long sentence. And another" } we can have , in your implementation { "description":[ "One long sentence.", " And another long sentence. ", "And another" ] }

as an easy to read array of sentences. But will be somehow serialized back as string, not as string[]

[dualbios]

Sorry, I have missed the core of your idea. Could you please describe the data schema of the database and EF? In other words, what is a template of data stored in a database, and what is a template of data in EntityFramework?