---
description: "Implement the EF Core entity classes and enums for Account, AccountYear, Entry, TransferLink, and EntryType"
agent: "agent"
---

# Step 2: Domain Model

Implement the domain entities under `src/Duempelkas.Domain`.

## Entities

### Account (`Entities/Account.cs`)
- `int Id` (PK)
- `string Name` (required, max 200)
- `DateTime CreatedUtc`
- `ICollection<AccountYear> AccountYears` (navigation)

### AccountYear (`Entities/AccountYear.cs`)
- `int Id` (PK)
- `int AccountId` (FK → Account)
- `int Year` (e.g. 2026)
- `decimal OpeningBalance` — explicit carryover from the previous year, user-entered
- `DateTime CreatedUtc`
- `Account Account` (navigation)
- `ICollection<Entry> Entries` (navigation)

Constraint: unique on `(AccountId, Year)`.

### Entry (`Entities/Entry.cs`)
- `int Id` (PK)
- `int AccountYearId` (FK → AccountYear)
- `EntryType Type` — Income or Expense
- `DateTime Date`
- `string Title` (required, max 500)
- `decimal Amount` — always positive; sign determined by Type
- `int? TransferLinkId` (FK → TransferLink, nullable)
- `DateTime CreatedUtc`
- `AccountYear AccountYear` (navigation)
- `TransferLink? TransferLink` (navigation)

### TransferLink (`Entities/TransferLink.cs`)
- `int Id` (PK)
- `int SourceEntryId` (FK → Entry, unique)
- `int TargetEntryId` (FK → Entry, unique)
- `string? Note`
- `DateTime CreatedUtc`
- `Entry SourceEntry` (navigation)
- `Entry TargetEntry` (navigation)

Invariants enforced at service level:
- SourceEntry.Type must be Expense
- TargetEntry.Type must be Income
- Both share the same Amount and Date

### EntryType (`Enums/EntryType.cs`)
```csharp
public enum EntryType { Income = 0, Expense = 1 }
```

## Conventions
- Use `decimal` for all monetary amounts.
- Store amounts as positive; behaviour (add/subtract) comes from `EntryType`.
- File-scoped namespaces, nullable enabled.