Keep a bill's category and payment mode when it is updated

PayForMe.xcodeproj/project.pbxproj

@@ -7,6 +7,7 @@
 	objects = {
 
 /* Begin PBXBuildFile section */
+		CF5E3E2FB8C03CEFE2577197 /* BillCategoryPreservationTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = 8B9248AB9EA3BF11E8FC22AC /* BillCategoryPreservationTests.swift */; };
 		481FB4FB23E964C8003BD108 /* ProjectManager.swift in Sources */ = {isa = PBXBuildFile; fileRef = 481FB4FA23E964C8003BD108 /* ProjectManager.swift */; };
 		481FB4FD23EAD78F003BD108 /* AddProjectManualViewModel.swift in Sources */ = {isa = PBXBuildFile; fileRef = 481FB4FC23EAD78F003BD108 /* AddProjectManualViewModel.swift */; };
 		489995DA23F6EC6F008B7E38 /* OnboardingView.swift in Sources */ = {isa = PBXBuildFile; fileRef = 489995D923F6EC6F008B7E38 /* OnboardingView.swift */; };
@@ -88,6 +89,7 @@
 /* End PBXContainerItemProxy section */
 
 /* Begin PBXFileReference section */
+		8B9248AB9EA3BF11E8FC22AC /* BillCategoryPreservationTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = BillCategoryPreservationTests.swift; sourceTree = "<group>"; };
 		0865D2DAF541B24078D1DC41 /* TestHelpers.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = TestHelpers.swift; sourceTree = "<group>"; };
 		143F41EE78329905894C8A29 /* BalanceCalculationTests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = BalanceCalculationTests.swift; sourceTree = "<group>"; };
 		316FBA7C0CE4ABB51EC2A121 /* NetworkRequestTests.swift */ = {isa = PBXFileReference; includeInIndex = 1; lastKnownFileType = sourcecode.swift; path = NetworkRequestTests.swift; sourceTree = "<group>"; };
@@ -227,6 +229,7 @@
 		654754E22528B29F00A82EB6 /* PayForMeTests */ = {
 			isa = PBXGroup;
 			children = (
+				8B9248AB9EA3BF11E8FC22AC /* BillCategoryPreservationTests.swift */,
 				654754E32528B29F00A82EB6 /* AddProjectManuallyTests.swift */,
 				654754E52528B29F00A82EB6 /* Info.plist */,
 				6523FC4B25580EEF00BCD843 /* UrlExtensionsTests.swift */,
@@ -543,6 +546,7 @@
 			isa = PBXSourcesBuildPhase;
 			buildActionMask = 2147483647;
 			files = (
+				CF5E3E2FB8C03CEFE2577197 /* BillCategoryPreservationTests.swift in Sources */,
 				654754E42528B29F00A82EB6 /* AddProjectManuallyTests.swift in Sources */,
 				6523FC4C25580EEF00BCD843 /* UrlExtensionsTests.swift in Sources */,
 				57BC8D25B91CE40F4669650C /* TestHelpers.swift in Sources */,

PayForMe/Model/Bill.swift

@@ -16,6 +16,11 @@ struct Bill: Codable, Identifiable, Hashable {
     var owers: [Person]
     var `repeat`: String?
     var lastchanged: Int?
+    /// Cospend category id. Built-in categories are negative, custom ones
+    /// positive, `0`/`nil` means uncategorised. iHateMoney never sends it.
+    var categoryid: Int?
+    /// Cospend payment mode ("n" = none, "c" = card, "b" = cash, ...).
+    var paymentmode: String?
 
     func paramsFor(_ backend: ProjectBackend) -> [String: Any] {
         var dict: [String: Any] = [
@@ -26,8 +31,12 @@ struct Bill: Codable, Identifiable, Hashable {
         ]
         if backend == .cospend {
             dict["payed_for"] = owers.map { $0.id.description }.joined(separator: ",")
-            dict["paymentmode"] = "n"
-            dict["categoryid"] = "0"
+            // Round-trip whatever the server told us instead of resetting to the
+            // defaults. Updating a bill previously wiped its category and
+            // payment mode, because the fields were never decoded in the first
+            // place and were hardcoded back to the defaults on every PUT.
+            dict["paymentmode"] = paymentmode ?? "n"
+            dict["categoryid"] = (categoryid ?? 0).description
 
             if let rep = self.repeat {
                 dict["repeat"] = rep

PayForMe/Views/BillDetail/BillDetailViewModel.swift

@@ -68,7 +68,18 @@ class BillDetailViewModel: ObservableObject {
 
         let actualOwers = povm.actualOwers()
 
-        return Bill(id: billID, amount: doubleAmount, what: topic, date: date, payer_id: selectedPayer, owers: actualOwers, repeat: currentProject.backend == .cospend ? "n" : nil, lastchanged: 0)
+        // The edit form has no category or payment-mode field, so both are
+        // carried over from the bill being edited rather than dropped.
+        return Bill(id: billID,
+                    amount: doubleAmount,
+                    what: topic,
+                    date: date,
+                    payer_id: selectedPayer,
+                    owers: actualOwers,
+                    repeat: currentProject.backend == .cospend ? "n" : nil,
+                    lastchanged: 0,
+                    categoryid: currentBill.categoryid,
+                    paymentmode: currentBill.paymentmode)
     }
 
     func prefillData() {

PayForMeTests/BillCategoryPreservationTests.swift

@@ -0,0 +1,123 @@
+//
+//  BillCategoryPreservationTests.swift
+//  PayForMeTests
+//
+//  Regression tests for a data-loss bug: editing any bill in a Cospend project
+//  reset its category and payment mode on the server.
+//
+//  Two things went wrong together. `Bill` never decoded `categoryid` or
+//  `paymentmode`, so the values were lost the moment a bill arrived; and
+//  `paramsFor(_:)` hardcoded `"categoryid": "0"` and `"paymentmode": "n"` into
+//  every PUT. Changing a bill's amount therefore also silently moved it to
+//  "uncategorised" — and the category statistics in Cospend degraded a little
+//  more with each edit.
+//
+
+import XCTest
+@testable import PayForMe
+
+final class BillCategoryPreservationTests: XCTestCase {
+
+    private let member = Person(id: 1, weight: 1, name: "Alice", activated: true)
+    private var testDate: Date { DateFormatter.cospend.date(from: "2026-05-14")! }
+
+    private func decoder() -> JSONDecoder {
+        let decoder = JSONDecoder()
+        decoder.dateDecodingStrategy = .formatted(DateFormatter.cospend)
+        return decoder
+    }
+
+    // MARK: Decoding
+
+    func testCategoryAndPaymentModeAreDecoded() throws {
+        let json = """
+        [{"id":42,"amount":12.5,"what":"Grocery run","date":"2026-05-14","payer_id":1,
+          "owers":[],"repeat":"n","categoryid":-1,"paymentmode":"c"}]
+        """.data(using: .utf8)!
+
+        let bills = try decoder().decode([Bill].self, from: json)
+        XCTAssertEqual(bills.first?.categoryid, -1)
+        XCTAssertEqual(bills.first?.paymentmode, "c")
+    }
+
+    /// iHateMoney never sends either field, and older Cospend payloads may not
+    /// either — their absence must not fail the whole response.
+    func testPayloadWithoutTheFieldsStillDecodes() throws {
+        let json = """
+        [{"id":7,"amount":12.5,"what":"Tea","date":"2026-05-04","payer_id":1,
+          "owers":[],"repeat":"n"}]
+        """.data(using: .utf8)!
+
+        let bills = try decoder().decode([Bill].self, from: json)
+        XCTAssertEqual(bills.count, 1)
+        XCTAssertNil(bills.first?.categoryid)
+        XCTAssertNil(bills.first?.paymentmode)
+    }
+
+    // MARK: Request parameters
+
+    func testParamsPreserveCategoryAndPaymentMode() {
+        let bill = Bill(id: 9, amount: 5, what: "Rent", date: testDate, payer_id: 1,
+                        owers: [member], repeat: "n", lastchanged: nil,
+                        categoryid: -3, paymentmode: "c")
+
+        let params = bill.paramsFor(.cospend)
+        XCTAssertEqual(params["categoryid"] as? String, "-3",
+                       "editing a bill must not move it to 'uncategorised'")
+        XCTAssertEqual(params["paymentmode"] as? String, "c")
+    }
+
+    /// A brand new bill has neither, and the server expects the documented
+    /// defaults rather than an empty value.
+    func testParamsFallBackToCospendDefaults() {
+        let bill = Bill(id: -1, amount: 5, what: "New", date: testDate, payer_id: 1,
+                        owers: [member], repeat: "n")
+
+        let params = bill.paramsFor(.cospend)
+        XCTAssertEqual(params["categoryid"] as? String, "0")
+        XCTAssertEqual(params["paymentmode"] as? String, "n")
+    }
+
+    /// iHateMoney has no such concepts; sending them would be noise.
+    func testIHateMoneyParamsCarryNeitherField() {
+        let bill = Bill(id: 9, amount: 5, what: "Rent", date: testDate, payer_id: 1,
+                        owers: [member], repeat: "n", lastchanged: nil,
+                        categoryid: -3, paymentmode: "c")
+
+        let params = bill.paramsFor(.iHateMoney)
+        XCTAssertNil(params["categoryid"])
+        XCTAssertNil(params["paymentmode"])
+    }
+
+    // MARK: The edit path
+
+    /// The regression itself: load a categorised bill, edit it through the
+    /// detail view model, and the outgoing parameters must still carry the
+    /// original category.
+    func testEditingABillKeepsItsCategory() throws {
+        let json = """
+        [{"id":42,"amount":12.5,"what":"Grocery run","date":"2026-05-14","payer_id":1,
+          "owers":[],"repeat":"n","categoryid":-1,"paymentmode":"c"}]
+        """.data(using: .utf8)!
+        let original = try decoder().decode([Bill].self, from: json)[0]
+
+        let viewModel = BillDetailViewModel(currentBill: original)
+        let project = Project(name: "test", password: "pw", token: "tok",
+                              backend: .cospend, url: URL(string: "https://test.de")!,
+                              projectId: "test")
+        project.members = [member.id: member]
+        viewModel.currentProject = project
+        viewModel.selectedPayer = member.id
+        viewModel.topic = "Grocery run"
+        // The user changes only the amount.
+        viewModel.amount = "18.00"
+        viewModel.povm.members = [member]
+        viewModel.povm.isOwing = [true]
+
+        let edited = try XCTUnwrap(viewModel.createBill())
+        XCTAssertEqual(edited.amount, 18.0, accuracy: 0.001, "the edit itself still applies")
+        XCTAssertEqual(edited.categoryid, -1, "the category must survive the edit")
+        XCTAssertEqual(edited.paymentmode, "c", "the payment mode must survive the edit")
+        XCTAssertEqual(edited.paramsFor(.cospend)["categoryid"] as? String, "-1")
+    }
+}