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Backend Gateway ↔ Child Association

Association authority is server-side. Firmware consumes only the effective current state through onboarding/sync.

Canonical model​

GatewayChildAssociation in IOT.App:

Id                    Guid
ProjectId             Guid      (real relational FK)
GatewayDeviceSerialId Guid      (cross-service reference)
ChildDeviceSerialId   Guid      (cross-service reference)
BoundAtUtc             DateTime
UnboundAtUtc           DateTime?
UnbindReason           bounded string
ConcurrencyToken       rowversion

GatewayDeviceSerialId and ChildDeviceSerialId reference IOT.Center.DeviceSerial.Id. Because service databases are separate, these are not cross-database SQL foreign keys.

Active association​

UnboundAtUtc == null

A SQL Server filtered unique index guarantees at most one active Gateway per Child:

builder.HasIndex(x => x.ChildDeviceSerialId)
    .IsUnique()
    .HasFilter("[UnboundAtUtc] IS NULL");

Routing​

Child DeviceSerial
→ active GatewayChildAssociation
→ GatewayDeviceSerialId
→ Physical Gateway DeviceSerial
→ dn/cmd

Never route using the latest historical row.

Delete and reassociation​

  • unbind closes the row by setting UnboundAtUtc/reason; it does not delete history
  • reassociation closes the old row and creates a new one
  • deleting a Gateway never deletes Child identity/history
  • local Gateway factory reset does not mutate Backend association by itself