Actual idempotent Insert or Update Migration Script for EF Core

CreateOrUpdateOperation.cs

public class CreateOrUpdateOperation : MigrationOperation
    {
        public EntityWithName Entity { get; }
        public Type EntityType { get; }

        public CreateOrUpdateOperation(EntityWithName entity, Type entityType)
        {
            Entity = entity;
            EntityType = entityType;
        }
    }

    public class CreateOrUpdateOperation<T> : CreateOrUpdateOperation where T : EntityWithName
    {

        public CreateOrUpdateOperation(T entity) : base(entity, typeof(T))
        {
        }
    }

DataContext.cs

        protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        {
            base.OnConfiguring(optionsBuilder);
            optionsBuilder.ReplaceService<IMigrationsSqlGenerator, TicketToolMigrationsSqlGenerator>();
        }

Migration.cs

    public partial class Test : Migration
    {
        protected override void Up(MigrationBuilder migrationBuilder)
        {
            migrationBuilder.CreateOrUpdate(YourEntity.SeedData);
        }

        protected override void Down(MigrationBuilder migrationBuilder)
        {

        }
    }

MigrationExtensions.cs

    public static class MigrationExtensions
    {
        public static MigrationBuilder CreateOrUpdate<T>(this MigrationBuilder builder, params T[] entities) where T : EntityWithName
        {
            foreach(var entity in entities)
                builder.Operations.Add(new CreateOrUpdateOperation<T>(entity));

            return builder;
        }
    }

MigrationsSqlGenerator.cs

public class TicketToolMigrationsSqlGenerator : SqlServerMigrationsSqlGenerator
    {
        public TicketToolMigrationsSqlGenerator([NotNull] MigrationsSqlGeneratorDependencies dependencies, 
                                                [NotNull] IMigrationsAnnotationProvider migrationsAnnotations) 
            : base(dependencies, migrationsAnnotations)
        {
        }

        protected override void Generate(MigrationOperation operation, IModel model, MigrationCommandListBuilder builder)
        {
            switch (operation)
            {
                    case CreateOrUpdateOperation createOrUpdate:
                        GenerateCreateOrUpdate(createOrUpdate, model, builder);
                        break;
                    default:
                        base.Generate(operation, model, builder);
                        break;
            }
        }

        private void GenerateCreateOrUpdate(CreateOrUpdateOperation operation, IModel model, MigrationCommandListBuilder builder)
        {
            var sqlHelper = Dependencies.SqlGenerationHelper;
            var entityType = model.FindEntityType(operation.EntityType);
            var relationalType = entityType.Relational();
            var primaryKey = entityType.GetProperties().Single(x => x.IsPrimaryKey()).Relational().ColumnName;

            var stringMapping = Dependencies.TypeMappingSource.FindMapping(typeof(string));
            var longMapping = Dependencies.TypeMappingSource.FindMapping(typeof(long));

            var schema = string.IsNullOrWhiteSpace(relationalType.Schema) ? sqlHelper.DelimitIdentifier("dbo") : sqlHelper.DelimitIdentifier(relationalType.Schema);
            var table = sqlHelper.DelimitIdentifier(relationalType.TableName);
            
            var idColumn = sqlHelper.DelimitIdentifier(primaryKey);
            var nameColumn = sqlHelper.DelimitIdentifier(nameof(EntityWithName.Name));

            var idValue = longMapping.GenerateSqlLiteral(operation.Entity.Id);
            var nameValue = stringMapping.GenerateSqlLiteral(operation.Entity.Name);

            builder.Append($"SET IDENTITY_INSERT {schema}.{table} ON")
                   .AppendLine(sqlHelper.StatementTerminator)
                   .AppendLine($"IF EXISTS (SELECT * FROM {schema}.{table} WHERE {idColumn} = {idValue})")
                   .AppendLine($"\tUPDATE {schema}.{table} SET {nameColumn} = {nameValue} WHERE {idColumn} = {idValue}")
                   .AppendLine($"ELSE")
                   .Append($"\tINSERT INTO {schema}.{table} ({idColumn}, {nameColumn}) VALUES ({idValue}, {nameValue})")
                   .AppendLine(sqlHelper.StatementTerminator)
                   .Append($"SET IDENTITY_INSERT {schema}.{table} ON")
                   .Append(sqlHelper.StatementTerminator)
                   .EndCommand();
        }
    }