How to store JSON in an entity field with EF Core?

Going to answer this one differently.

Ideally the domain model should have no idea how data is stored. Adding backing fields and extra [NotMapped] properties is actually coupling your domain model to your infrastructure.

Remember - your domain is king, and not the database. The database is just being used to store parts of your domain.

Instead you can use EF Core's HasConversion() method on the EntityTypeBuilder object to convert between your type and JSON.

Given these 2 domain models:

public class Person
{
    public int Id { get; set; }

    [Required]
    [MaxLength(50)]
    public string FirstName { get; set; }

    [Required]
    [MaxLength(50)]
    public string LastName { get; set; }

    [Required]
    public DateTime DateOfBirth { get; set; }

    public IList<Address> Addresses { get; set; }      
}

public class Address
{
    public string Type { get; set; }
    public string Company { get; set; }
    public string Number { get; set; }
    public string Street { get; set; }
    public string City { get; set; }
}

I have only added attributes that the domain is interested in - and not details that the DB would be interested in; I.E there is no [Key].

My DbContext has the following IEntityTypeConfiguration for the Person:

public class PersonsConfiguration : IEntityTypeConfiguration<Person>
{
    public void Configure(EntityTypeBuilder<Person> builder)
    {
        // This Converter will perform the conversion to and from Json to the desired type
        builder.Property(e => e.Addresses).HasConversion(
            v => JsonConvert.SerializeObject(v, new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore }),
            v => JsonConvert.DeserializeObject<IList<Address>>(v, new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore }));
    }
}

With this method you can completely decouple your domain from your infrastructure. No need for all the backing field & extra properties.

The key to making the the Change Tracker function correctly is to implement a ValueComparer as well as a ValueConverter. Below is an extension to implement such:

public static class ValueConversionExtensions
{
    public static PropertyBuilder<T> HasJsonConversion<T>(this PropertyBuilder<T> propertyBuilder) where T : class, new()
    {
        ValueConverter<T, string> converter = new ValueConverter<T, string>
        (
            v => JsonConvert.SerializeObject(v),
            v => JsonConvert.DeserializeObject<T>(v) ?? new T()
        );

        ValueComparer<T> comparer = new ValueComparer<T>
        (
            (l, r) => JsonConvert.SerializeObject(l) == JsonConvert.SerializeObject(r),
            v => v == null ? 0 : JsonConvert.SerializeObject(v).GetHashCode(),
            v => JsonConvert.DeserializeObject<T>(JsonConvert.SerializeObject(v))
        );

        propertyBuilder.HasConversion(converter);
        propertyBuilder.Metadata.SetValueConverter(converter);
        propertyBuilder.Metadata.SetValueComparer(comparer);
        propertyBuilder.HasColumnType("jsonb");

        return propertyBuilder;
    }
}

Example of how this works.

public class Person
{
    public int Id { get; set; }

    [Required]
    [MaxLength(50)]
    public string FirstName { get; set; }

    [Required]
    [MaxLength(50)]
    public string LastName { get; set; }

    [Required]
    public DateTime DateOfBirth { get; set; }

    public List<Address> Addresses { get; set; }      
}

public class Address
{
    public string Type { get; set; }
    public string Company { get; set; }
    public string Number { get; set; }
    public string Street { get; set; }
    public string City { get; set; }
}

public class PersonsConfiguration : IEntityTypeConfiguration<Person>
{
    public void Configure(EntityTypeBuilder<Person> builder)
    {
        // This Converter will perform the conversion to and from Json to the desired type
        builder.Property(e => e.Addresses).HasJsonConversion<IList<Address>>();
    }
}

This will make the ChangeTracker function correctly.

For those using EF 2.1 there is a nice little NuGet package EfCoreJsonValueConverter that makes it pretty simple.

using Innofactor.EfCoreJsonValueConverter;
using Microsoft.EntityFrameworkCore;
using Microsoft.EntityFrameworkCore.Metadata.Builders;

public class Campaign
{
    [Key]
    public Guid Id { get; set; }

    [Required]
    [MaxLength(50)]
    public string Name { get; set; }

    public JObject ExtendedData { get; set; }
}

public class CampaignConfiguration : IEntityTypeConfiguration<Campaign> 
{
    public void Configure(EntityTypeBuilder<Campaign> builder) 
    {
        builder
            .Property(application => application.ExtendedData)
            .HasJsonValueConversion();
    }
}

I have made a solution based on Robert Raboud's contribution. The change made by me is that my implementation uses a HasJsonConversion method that depends on the System.Text.Json package rather than Newtonsofts library:

    public static PropertyBuilder<T> HasJsonConversion<T>(this PropertyBuilder<T> propertyBuilder) where T : class, new()
    {
        var options = new JsonSerializerOptions
        {
            PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
            WriteIndented = true,
            AllowTrailingCommas = true,
            PropertyNameCaseInsensitive = true
        };

        ValueConverter<T, string> converter = new ValueConverter<T, string>
        (
            v => JsonSerializer.Serialize(v, options),
            v => JsonSerializer.Deserialize<T>(v, options) ?? new T()
        );

        ValueComparer<T> comparer = new ValueComparer<T>
        (
            (l, r) => JsonSerializer.Serialize(l, options) == JsonSerializer.Serialize(r, options),
            v => v == null ? 0 : JsonSerializer.Serialize(v, options).GetHashCode(),
            v => JsonSerializer.Deserialize<T>(JsonSerializer.Serialize(v, options), options)
        );

        propertyBuilder.HasConversion(converter);
        propertyBuilder.Metadata.SetValueConverter(converter);
        propertyBuilder.Metadata.SetValueComparer(comparer);
        propertyBuilder.HasColumnType("LONGTEXT");

        return propertyBuilder;
    }

Note also that this implementation expects for the column to be LONGTEXT since I am using a MySQL setup.

Here's something I used

Model

public class FacilityModel 
{
    public string Name { get; set; } 
    public JObject Values { get; set; } 
}

Entity

[Table("facility", Schema = "public")]
public class Facility 
{
     public string Name { get; set; } 
     public Dictionary<string, string> Values { get; set; } = new Dictionary<string, string>();
}

Mapping

this.CreateMap<Facility, FacilityModel>().ReverseMap();

DBContext

base.OnModelCreating(builder); 
        builder.Entity<Facility>()
        .Property(b => b.Values)
        .HasColumnType("jsonb")
        .HasConversion(
        v => JsonConvert.SerializeObject(v),
        v => JsonConvert.DeserializeObject<Dictionary<string, string>>(v));

For developers, who work with EF Core 3.1 and meet such error ("The entity type 'XXX' requires a primary key to be defined. If you intended to use a keyless entity type call 'HasNoKey()'.") the solution is:

Move .HasConversion() method with it's lambda from: OrderConfiguration : IEntityTypeConfiguration<T> to OnModelCreating(ModelBuilder modelBuilder) in your DataContext.

// DbContext

  protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
            var entityTypes = modelBuilder.Model.GetEntityTypes();
            foreach (var entityType in entityTypes)
            {
                foreach (var property in entityType.ClrType.GetProperties().Where(x => x != null && x.GetCustomAttribute<HasJsonConversionAttribute>() != null))
                {
                    modelBuilder.Entity(entityType.ClrType)
                        .Property(property.PropertyType, property.Name)
                        .HasJsonConversion();
                }
            }

            base.OnModelCreating(modelBuilder);
        }

Create an attribute to handle the properties of the entities.

public class HasJsonConversionAttribute : System.Attribute
    {

    }

Create extention class to find Josn properties

    public static class ValueConversionExtensions
    {
        public static PropertyBuilder HasJsonConversion(this PropertyBuilder propertyBuilder)
        {
            ParameterExpression parameter1 = Expression.Parameter(propertyBuilder.Metadata.ClrType, "v");

            MethodInfo methodInfo1 = typeof(Newtonsoft.Json.JsonConvert).GetMethod("SerializeObject", types: new Type[] { typeof(object) });
            MethodCallExpression expression1 = Expression.Call(methodInfo1 ?? throw new Exception("Method not found"), parameter1);

            ParameterExpression parameter2 = Expression.Parameter(typeof(string), "v");
            MethodInfo methodInfo2 = typeof(Newtonsoft.Json.JsonConvert).GetMethod("DeserializeObject", 1, BindingFlags.Static | BindingFlags.Public, Type.DefaultBinder, CallingConventions.Any, types: new Type[] { typeof(string) }, null)?.MakeGenericMethod(propertyBuilder.Metadata.ClrType) ?? throw new Exception("Method not found");
            MethodCallExpression expression2 = Expression.Call(methodInfo2, parameter2);

            var converter = Activator.CreateInstance(typeof(ValueConverter<,>).MakeGenericType(typeof(List<AttributeValue>), typeof(string)), new object[]
                {
                    Expression.Lambda( expression1,parameter1),
                    Expression.Lambda( expression2,parameter2),
                    (ConverterMappingHints) null
                });

            propertyBuilder.HasConversion(converter as ValueConverter);

            return propertyBuilder;
        }
    }

Entity example

 public class Attribute
    {
        [Key, DatabaseGenerated(DatabaseGeneratedOption.Identity)]
        public int Id { get; set; }
        public string Name { get; set; }

        [HasJsonConversion]
        public List<AttributeValue> Values { get; set; }
    }

    public class AttributeValue
    {
        public string Value { get; set; }
        public IList<AttributeValueTranslation> Translations { get; set; }
    }

    public class AttributeValueTranslation
    {
        public string Translation { get; set; }

        public string CultureName { get; set; }
    }

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For those who are working on entity framework core 5.0 and above. below can work if you are getting error like below

The entity type '{EntityName}' requires a primary key to be defined. If you intended to use a keyless entity type, call 'HasNoKey' in 'OnModelCreating'. The error is asking to define a primary key on the model

Try this

protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder
            .Entity<Person>(
                eb =>
                {
                    eb.Property(p => p.Addresses).HasConversion(

        v => JsonConvert.SerializeObject(v, new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore }),
        v => JsonConvert.DeserializeObject<IList<Address>>(v, new JsonSerializerSettings { NullValueHandling = NullValueHandling.Ignore })
                        );
                });
    }

The comment by @Métoule:

Be careful with this approach: EF Core marks an entity as modified only if the field is assigned to. So if you use person.Addresses.Add, the entity won't be flagged as updated; you'll need to call the property setter person.Addresses = updatedAddresses.

made me take a different approach so that this fact is obvious: use Getter and Setter methods, rather than a property.

public void SetExtendedData(JObject extendedData) {
    ExtendedData = JsonConvert.SerializeObject(extendedData);
    _deserializedExtendedData = extendedData;
}

//just to prevent deserializing more than once unnecessarily
private JObject _deserializedExtendedData;

public JObject GetExtendedData() {
    if (_extendedData != null) return _deserializedExtendedData;
    _deserializedExtendedData = string.IsNullOrEmpty(ExtendedData) ? null : JsonConvert.DeserializeObject<JObject>(ExtendedData);
    return _deserializedExtendedData;
}

You could theoretically do this:

campaign.GetExtendedData().Add(something);

But it's much more clear that That Doesn't Do What You Think It Does™.

If you're using database-first and using some kind of class auto-generator for EF, then the classes will usually be declared as partial, so you can add this stuff in a separate file that won't get blown away the next time you update your classes from your database.