[Entity Framework core] Seeding data

 Entity Framework Core   Seed 

▌Introduction

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This article shows how to seed (initialize data) with Entity framework core and some key points.

▌Environment

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▋ASP.NET Core 2.2.101

▋EF Core 2.2.4

▌Implement

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▋Create ModelBuilderExtensions

In the Data Access project, create the ModelBuilderExtensions and use EntityTypeBuilder<TEntity>.HasData(TEntity[]) to have seed data as following,

▋ModelBuilderExtensions.cs

public static class ModelBuilderExtensions

{

public static void Seed(this ModelBuilder modelBuilder)

        {

            modelBuilder.Entity<SupportedLanguage>()

               .HasData(

                   new MapLang {

Id = 1,

Name = "繁體中文",

NameEn = "Traditional Chinese",

NameCn = "繁體中文" },

                   new MapLang {

Id = 2,

Name = "简体中文",

NameEn = "Simplified Chinese",

NameCn = "简体中文" },

                   new MapLang {

Id = 3,

Name = "英文",

NameEn = "English",

NameCn = "英文" });

        }

 }

Notice that even if the Primary key (For example: “Id”) is an Identity/Sequence column and has DataAnnotation, [DatabaseGenerated(DatabaseGeneratedOption.Identity)], to make sure that the seed data is in database, EF core needs the specified value of the Identity/Sequence column when seeding data.

When the HasData method is used, EF Core automatically generates SET IDENTITY INSERT ON for the relevant table, and the set it to OFF once the seeding has completed. So that will make the table’s Sequence not updated.

For example, the latest sequence of table, SupportedLanguages, is still be 1 after seeding data.

So we have to manually update/reset the Sequence.

1.  Generate migration files by

dotnet ef migrations add <migration_name>

2.  Update the migration

Find the migration file, such as 20190603123102_InitCreate.cs. Write down the update/reset Sequence sql in the end of the Up function.

public partial class InitCreate : Migration

    {

        protected override void Up(MigrationBuilder migrationBuilder)

        {

            //...

           

            // Sql server

            migrationBuilder.Sql(@"

ALTER SEQUENCE SupportedLanguages RESTART WITH 4;");

            // Postgres

            migrationBuilder.Sql(@"

ALTER SEQUENCE ""SupportedLanguages_Id_seq"" RESTART WITH 4;");

       

        }

    }

Then the Sequence will keep the right latest number after updating the migration.

▌Reference

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▋Data Seeding