@startuml update-username
!include ../../../pipeline/diagrams/biergarten-weizen-theme.puml
skinparam sequenceMessageAlign center
skinparam maxMessageSize 200

title User Authentication Flow - Update Username

actor User
participant "API\nController" as API
participant "MediatR pipeline\n(ValidationBehavior)" as Mediator
box "Handler Layer" #F1F3EA
participant "UpdateUsernameHandler" as UpdateUsernameHandler
participant "UserManager<ApplicationUser>" as UserMgr
end box
box "Repository / Cross-slice" #DBEEDD
participant "DapperUserStore\n(IUserStore/IUserPasswordStore/\nIUserEmailStore)" as UserStore
end box
database "SQL Server" as DB

note over UserMgr
  ASP.NET Core Identity's UserManager<TUser>
  replaces the deleted AuthRepository: handlers
  no longer issue SQL or call Argon2 directly,
  they delegate to UserManager, which in turn
  calls DapperUserStore (persistence) and
  Argon2PasswordHasher (IPasswordHasher<TUser>).
end note

User -> API: PATCH /api/auth/username\n{newUsername}\n(requires Bearer access token)
activate API

API -> Mediator: Send(UpdateUsernameCommand(\n  GetAuthenticatedUserId(), newUsername))
activate Mediator

note right of Mediator
  ValidationBehavior runs UpdateUsernameValidator
  (same format rules as registration)
end note

alt Validation fails
  Mediator -> API: throws FluentValidation.ValidationException
  API -> User: 400 Bad Request
else Validation succeeds

  Mediator -> UpdateUsernameHandler: Handle(command)
  activate UpdateUsernameHandler

  UpdateUsernameHandler -> UserMgr: FindByIdAsync(userAccountId)
  activate UserMgr
  UserMgr -> UserStore: FindByIdAsync(userAccountId)
  activate UserStore
  UserStore -> DB: SELECT ...\nFROM dbo.UserAccount\nWHERE UserAccountID = @Id
  activate DB
  DB --> UserStore: row or null
  deactivate DB
  UserStore --> UserMgr: ApplicationUser or null
  deactivate UserStore
  UserMgr --> UpdateUsernameHandler: ApplicationUser or null
  deactivate UserMgr

  alt Account not found
    UpdateUsernameHandler -> Mediator: throw NotFoundException
    Mediator -> API: propagate
    API -> User: 404 Not Found
  else Account found

    UpdateUsernameHandler -> UserMgr: SetUserNameAsync(user, newUsername)
    activate UserMgr
    UserMgr -> UserStore: FindByNameAsync(newUsername)
    note right of UserMgr
      Uniqueness check before the UPDATE.
    end note
    UserMgr -> UserStore: UpdateAsync(user)
    UserStore -> DB: UPDATE dbo.UserAccount\nSET Username = @UserName, ...
    UserStore --> UserMgr: IdentityResult
    UserMgr --> UpdateUsernameHandler: IdentityResult
    deactivate UserMgr

    alt !result.Succeeded
      UpdateUsernameHandler -> Mediator: throw ConflictException\n(DuplicateUserName code ->\n"Username already exists")
      Mediator -> API: propagate
      API -> User: 409 Conflict
    else result.Succeeded
      UpdateUsernameHandler --> Mediator: UpdateUsernamePayload(\n  user.Id, user.UserName)
      deactivate UpdateUsernameHandler
      Mediator --> API: UpdateUsernamePayload
      deactivate Mediator
      API -> User: 200 OK\n{message: "Username updated successfully.",\npayload: {...}}
    end
  end
end
deactivate API

@enduml