Update-Database fails on foreign key conflict

I've an application which uses Entity Framework. It contains several migration files which do various database operations. Another user has been making updates directly to the database and now I can't get one of my migrations to apply when I run Update-Database. This is the error I get:

PM> Update-Database Build started... Build succeeded. Failed executing DbCommand (56ms) [Parameters=[], CommandType='Text', CommandTimeout='30'] ALTER TABLE [Balance1Part] ADD CONSTRAINT [FK_Balance1Part_Balance1_Balance1Id] FOREIGN KEY ([Balance1Id]) REFERENCES [Balance1] ([Id]) ON DELETE CASCADE; fail: Microsoft.EntityFrameworkCore.Database.Command[20102] Failed executing DbCommand (56ms) [Parameters=[], CommandType='Text', CommandTimeout='30'] ALTER TABLE [Balance1Part] ADD CONSTRAINT [FK_Balance1Part_Balance1_Balance1Id] FOREIGN KEY ([Balance1Id]) REFERENCES [Balance1] ([Id]) ON DELETE CASCADE; Microsoft.Data.SqlClient.SqlException (0x80131904): The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_Balance1Part_Balance1_Balance1Id". The conflict occurred in database "db-vrhit05042021", table "dbo.Balance1", column 'Id'. at Microsoft.Data.SqlClient.SqlConnection.OnError(SqlException exception, Boolean breakConnection, Action1 wrapCloseInAction) at Microsoft.Data.SqlClient.SqlInternalConnection.OnError(SqlException exception, Boolean breakConnection, Action1 wrapCloseInAction) at Microsoft.Data.SqlClient.TdsParser.ThrowExceptionAndWarning(TdsParserStateObject stateObj, Boolean callerHasConnectionLock, Boolean asyncClose) at Microsoft.Data.SqlClient.TdsParser.TryRun(RunBehavior runBehavior, SqlCommand cmdHandler, SqlDataReader dataStream, BulkCopySimpleResultSet bulkCopyHandler, TdsParserStateObject stateObj, Boolean& dataReady) at Microsoft.Data.SqlClient.SqlCommand.RunExecuteNonQueryTds(String methodName, Boolean isAsync, Int32 timeout, Boolean asyncWrite) at Microsoft.Data.SqlClient.SqlCommand.InternalExecuteNonQuery(TaskCompletionSource1 completion, Boolean sendToPipe, Int32 timeout, Boolean& usedCache, Boolean asyncWrite, Boolean inRetry, String methodName) at Microsoft.Data.SqlClient.SqlCommand.ExecuteNonQuery() at Microsoft.EntityFrameworkCore.Storage.RelationalCommand.ExecuteNonQuery(RelationalCommandParameterObject parameterObject) at Microsoft.EntityFrameworkCore.Migrations.MigrationCommand.ExecuteNonQuery(IRelationalConnection connection, IReadOnlyDictionary2 parameterValues) at Microsoft.EntityFrameworkCore.Migrations.Internal.MigrationCommandExecutor.ExecuteNonQuery(IEnumerable`1 migrationCommands, IRelationalConnection connection) at Microsoft.EntityFrameworkCore.Migrations.Internal.Migrator.Migrate(String targetMigration) at Microsoft.EntityFrameworkCore.Design.Internal.MigrationsOperations.UpdateDatabase(String targetMigration, String contextType) at Microsoft.EntityFrameworkCore.Design.OperationExecutor.UpdateDatabaseImpl(String targetMigration, String contextType) at Microsoft.EntityFrameworkCore.Design.OperationExecutor.UpdateDatabase.<>c__DisplayClass0_0.<.ctor>b__0() at Microsoft.EntityFrameworkCore.Design.OperationExecutor.OperationBase.Execute(Action action) ClientConnectionId:b538f2ff-0a95-4975-ab15-ed75eca72382 Error Number:547,State:0,Class:16 The ALTER TABLE statement conflicted with the FOREIGN KEY constraint "FK_Balance1Part_Balance1_Balance1Id". The conflict occurred in database "db-vrhit05042021", table "dbo.Balance1", column 'Id'.

The migration I'm looking to apply here should add the foreign key FK_Balance1Part_Balance1_Balance1Id and that foreign key doesn't seem to exist on the database. How can I resolve this issue?

I'm trying to do this on an Azure SQL Server instance.

Here's the migration:

public partial class AddIdcolumns : Migration
{
    protected override void Up(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.AddColumn<int>(
            name: "CalibrationId",
            table: "Symptoms",
            nullable: false,
            defaultValue: 0);

        migrationBuilder.AddColumn<int>(
            name: "Id",
            table: "Balance1",
            nullable: false,
            defaultValue: 0)
            .Annotation("SqlServer:Identity", "1, 1");

        migrationBuilder.AddPrimaryKey(
            name: "PK_Balance1",
            table: "Balance1",
            column: "Id");

        migrationBuilder.CreateIndex(
            name: "IX_Symptoms_CalibrationId",
            table: "Symptoms",
            column: "CalibrationId");

        migrationBuilder.CreateIndex(
            name: "IX_Balance1Part_Balance1Id",
            table: "Balance1Part",
            column: "Balance1Id");

        migrationBuilder.AddForeignKey(
            name: "FK_Balance1Part_Balance1_Balance1Id",
            table: "Balance1Part",
            column: "Balance1Id",
            principalTable: "Balance1",
            principalColumn: "Id",
            onDelete: ReferentialAction.Cascade);

        migrationBuilder.AddForeignKey(
            name: "FK_Symptoms_Calibration_CalibrationId",
            table: "Symptoms",
            column: "CalibrationId",
            principalTable: "Calibration",
            principalColumn: "Id",
            onDelete: ReferentialAction.Cascade);
    }

    protected override void Down(MigrationBuilder migrationBuilder)
    {
        migrationBuilder.DropForeignKey(
            name: "FK_Balance1Part_Balance1_Balance1Id",
            table: "Balance1Part");

        migrationBuilder.DropForeignKey(
            name: "FK_Symptoms_Calibration_CalibrationId",
            table: "Symptoms");

        migrationBuilder.DropIndex(
            name: "IX_Symptoms_CalibrationId",
            table: "Symptoms");

        migrationBuilder.DropIndex(
            name: "IX_Balance1Part_Balance1Id",
            table: "Balance1Part");

        migrationBuilder.DropPrimaryKey(
            name: "PK_Balance1",
            table: "Balance1");

        migrationBuilder.DropColumn(
            name: "CalibrationId",
            table: "Symptoms");

        migrationBuilder.DropColumn(
            name: "Id",
            table: "Balance1");
    }
}

Context:

public class VRHitContext : DbContext
{
    public VRHitContext(DbContextOptions<VRHitContext> options)
        : base(options)
    {

    }

    public DbSet<Organisation> Organisations { get; set; }
    public DbSet<ClientDeviceRegistration> ClientDeviceRegistrations { get; set; }

    public DbSet<Session> Sessions { get; set; }
    public DbSet<User> Users { get; set; }

    public DbSet<DataFile> DataFiles { get; set; }

    public DbSet<Symptoms> Symptoms { get; set; }
    public DbSet<Balance1> Balance1 { get; set; }
    public DbSet<Balance1Raw> Balance1Raw { get; set; }
    public DbSet<Balance2> Balance2 { get; set; }
    public DbSet<Balance2Raw> Balance2Raw { get; set; }
    public DbSet<Calibration> Calibration { get; set; }
    public DbSet<CogMotor1> CogMotor1 { get; set; }
    public DbSet<CogMotor1Raw> CogMotor1Raw { get; set; }
    public DbSet<CogMotor2> CogMotor2 { get; set; }
    public DbSet<CogMotor2Raw> CogMotor2Raw { get; set; }

    public DbSet<CogMotor3> CogMotor3 { get; set; }
    public DbSet<CogMotor3Raw> CogMotor3Raw { get; set; }

    public DbSet<Gait> Gait { get; set; }
    public DbSet<GaitRaw> GaitRaw { get; set; }

    public DbSet<Interception1> Interception1 { get; set; }
    public DbSet<Interception2> Interception2 { get; set; }
    public DbSet<InterceptionPlayerRaw> InterceptionPlayerRaw { get; set; }
    public DbSet<ManualDexterity> ManualDexterity { get; set; }
    public DbSet<ManualDexterityRaw> ManualDexterityRaw { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>().HasIndex(u => u.UserIdentifier).IsUnique();
        modelBuilder.Entity<Balance1Part>()
            .HasKey(o => new { o.Id, o.Balance1Id });

        // Set default decimal precision.
        foreach (var property in modelBuilder.Model.GetEntityTypes()
            .SelectMany(t => t.GetProperties())
            .Where(p => p.ClrType == typeof(decimal) || p.ClrType == typeof(decimal?)))
                    {
                        property.SetColumnType("decimal(38, 10)");
                    }


        // Set default for GUID primary keys.
        foreach (var property in modelBuilder.Model.GetEntityTypes()
            .SelectMany(t => t.GetProperties())
            .Where(p => (p.ClrType == typeof(Guid) || p.ClrType == typeof(Guid?)) && p.IsPrimaryKey()))
        {
            property.SetDefaultValueSql("NEWID()");
        }
    }
}

I would say that this error occurred because you tried to add a foreign key from Balance1Part.Balance1Id to Balance1.Id but some values in Balance1Part.Balance1Id don't match any in Balance1.Id. It should be impossible to create a relation that violates referential integrity. Try checking for missing keys in Balance1Part:

SELECT Balance1Id FROM Balance1Part
WHERE Balance1Id NOT IN
(SELECT Id from Balance1)

To resolve add migrationBuilder.Sql("DELETE FROM [Balance1Part]", true); to the migration.