diff --git a/costly-api/package-lock.json b/costly-api/package-lock.json
index 0337cff..fac45c7 100644
--- a/costly-api/package-lock.json
+++ b/costly-api/package-lock.json
@@ -16,6 +16,7 @@
         "@nestjs/typeorm": "^11.0.0",
         "class-transformer": "^0.5.1",
         "class-validator": "^0.14.3",
+        "express-session": "^1.18.2",
         "mysql2": "^3.16.0",
         "reflect-metadata": "^0.2.2",
         "rxjs": "^7.8.1",
@@ -31,6 +32,7 @@
         "@swc/cli": "^0.6.0",
         "@swc/core": "^1.10.7",
         "@types/express": "^5.0.0",
+        "@types/express-session": "^1.18.2",
         "@types/jest": "^29.5.14",
         "@types/node": "^22.10.7",
         "@types/supertest": "^6.0.2",
@@ -3362,6 +3364,16 @@
         "@types/send": "*"
       }
     },
+    "node_modules/@types/express-session": {
+      "version": "1.18.2",
+      "resolved": "https://registry.npmjs.org/@types/express-session/-/express-session-1.18.2.tgz",
+      "integrity": "sha512-k+I0BxwVXsnEU2hV77cCobC08kIsn4y44C3gC0b46uxZVMaXA04lSPgRLR/bSL2w0t0ShJiG8o4jPzRG/nscFg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/express": "*"
+      }
+    },
     "node_modules/@types/graceful-fs": {
       "version": "4.1.9",
       "resolved": "https://registry.npmjs.org/@types/graceful-fs/-/graceful-fs-4.1.9.tgz",
@@ -6368,6 +6380,46 @@
         "url": "https://opencollective.com/express"
       }
     },
+    "node_modules/express-session": {
+      "version": "1.18.2",
+      "resolved": "https://registry.npmjs.org/express-session/-/express-session-1.18.2.tgz",
+      "integrity": "sha512-SZjssGQC7TzTs9rpPDuUrR23GNZ9+2+IkA/+IJWmvQilTr5OSliEHGF+D9scbIpdC6yGtTI0/VhaHoVes2AN/A==",
+      "license": "MIT",
+      "dependencies": {
+        "cookie": "0.7.2",
+        "cookie-signature": "1.0.7",
+        "debug": "2.6.9",
+        "depd": "~2.0.0",
+        "on-headers": "~1.1.0",
+        "parseurl": "~1.3.3",
+        "safe-buffer": "5.2.1",
+        "uid-safe": "~2.1.5"
+      },
+      "engines": {
+        "node": ">= 0.8.0"
+      }
+    },
+    "node_modules/express-session/node_modules/cookie-signature": {
+      "version": "1.0.7",
+      "resolved": "https://registry.npmjs.org/cookie-signature/-/cookie-signature-1.0.7.tgz",
+      "integrity": "sha512-NXdYc3dLr47pBkpUCHtKSwIOQXLVn8dZEuywboCOJY/osA0wFSLlSawr3KN8qXJEyX66FcONTH8EIlVuK0yyFA==",
+      "license": "MIT"
+    },
+    "node_modules/express-session/node_modules/debug": {
+      "version": "2.6.9",
+      "resolved": "https://registry.npmjs.org/debug/-/debug-2.6.9.tgz",
+      "integrity": "sha512-bC7ElrdJaJnPbAP+1EotYvqZsb3ecl5wi6Bfi6BJTUcNowp6cvspg0jXznRTKDjm/E7AdgFBVeAPVMNcKGsHMA==",
+      "license": "MIT",
+      "dependencies": {
+        "ms": "2.0.0"
+      }
+    },
+    "node_modules/express-session/node_modules/ms": {
+      "version": "2.0.0",
+      "resolved": "https://registry.npmjs.org/ms/-/ms-2.0.0.tgz",
+      "integrity": "sha512-Tpp60P6IUJDTuOq/5Z8cdskzJujfwqfOTkrwIwj7IRISpnkJnT6SyJ4PCPnGMoFjC9ddhal5KVIYtAt97ix05A==",
+      "license": "MIT"
+    },
     "node_modules/express/node_modules/content-disposition": {
       "version": "1.0.1",
       "resolved": "https://registry.npmjs.org/content-disposition/-/content-disposition-1.0.1.tgz",
@@ -9072,6 +9124,15 @@
         "node": ">= 0.8"
       }
     },
+    "node_modules/on-headers": {
+      "version": "1.1.0",
+      "resolved": "https://registry.npmjs.org/on-headers/-/on-headers-1.1.0.tgz",
+      "integrity": "sha512-737ZY3yNnXy37FHkQxPzt4UZ2UWPWiCZWLvFZ4fu5cueciegX0zGPnrlY6bwRg4FdQOe9YU8MkmJwGhoMybl8A==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 0.8"
+      }
+    },
     "node_modules/once": {
       "version": "1.4.0",
       "resolved": "https://registry.npmjs.org/once/-/once-1.4.0.tgz",
@@ -9626,6 +9687,15 @@
         "url": "https://github.com/sponsors/sindresorhus"
       }
     },
+    "node_modules/random-bytes": {
+      "version": "1.0.0",
+      "resolved": "https://registry.npmjs.org/random-bytes/-/random-bytes-1.0.0.tgz",
+      "integrity": "sha512-iv7LhNVO047HzYR3InF6pUcUsPQiHTM1Qal51DcGSuZFBil1aBBWG5eHPNek7bvILMaYJ/8RU1e8w1AMdHmLQQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 0.8"
+      }
+    },
     "node_modules/randombytes": {
       "version": "2.1.0",
       "resolved": "https://registry.npmjs.org/randombytes/-/randombytes-2.1.0.tgz",
@@ -11456,6 +11526,18 @@
         "node": ">=8"
       }
     },
+    "node_modules/uid-safe": {
+      "version": "2.1.5",
+      "resolved": "https://registry.npmjs.org/uid-safe/-/uid-safe-2.1.5.tgz",
+      "integrity": "sha512-KPHm4VL5dDXKz01UuEd88Df+KzynaohSL9fBh096KWAxSKZQDI2uBrVqtvRM4rwrIrRRKsdLNML/lnaaVSRioA==",
+      "license": "MIT",
+      "dependencies": {
+        "random-bytes": "~1.0.0"
+      },
+      "engines": {
+        "node": ">= 0.8"
+      }
+    },
     "node_modules/uint8array-extras": {
       "version": "1.5.0",
       "resolved": "https://registry.npmjs.org/uint8array-extras/-/uint8array-extras-1.5.0.tgz",
diff --git a/costly-api/package.json b/costly-api/package.json
index 9175f18..63d40c1 100644
--- a/costly-api/package.json
+++ b/costly-api/package.json
@@ -27,6 +27,7 @@
     "@nestjs/typeorm": "^11.0.0",
     "class-transformer": "^0.5.1",
     "class-validator": "^0.14.3",
+    "express-session": "^1.18.2",
     "mysql2": "^3.16.0",
     "reflect-metadata": "^0.2.2",
     "rxjs": "^7.8.1",
@@ -42,6 +43,7 @@
     "@swc/cli": "^0.6.0",
     "@swc/core": "^1.10.7",
     "@types/express": "^5.0.0",
+    "@types/express-session": "^1.18.2",
     "@types/jest": "^29.5.14",
     "@types/node": "^22.10.7",
     "@types/supertest": "^6.0.2",
diff --git a/costly-api/src/app.module.ts b/costly-api/src/app.module.ts
index 12ffec2..6fd086f 100644
--- a/costly-api/src/app.module.ts
+++ b/costly-api/src/app.module.ts
@@ -6,16 +6,31 @@ import { DatabaseModule } from './database/database.module';
 import { GroupsModule } from './groups/groups.module';
 import { ExpensesModule } from './expenses/expenses.module';
 import { GroupEventsModule } from './group-events/group-events.module';
+import { TypeOrmModule } from '@nestjs/typeorm';
 
 @Module({
   imports: [ConfigModule.forRoot({
     envFilePath: '.env',
     isGlobal: true
   }),
-  DatabaseModule,
+  TypeOrmModule.forRootAsync({
+    imports: [ConfigModule],
+    inject: [ConfigService],
+    useFactory: (configService: ConfigService) => ({
+        type: 'mysql',
+        host: configService.get('DATABASE_HOST') || 'localhost',
+        port: parseInt(configService.get('DATABASE_PORT') || '0'),
+        username: configService.get('DATABASE_USER'),
+        password: configService.get('DATABASE_PASSWORD'),
+        database: configService.get('DATABASE_NAME'),
+        entities: [],
+        synchronize: (configService.get('DATABASE_SYNC')||'').toLowerCase() == 'true',
+    })
+  }),
   GroupsModule,
   ExpensesModule,
-  GroupEventsModule
+  GroupEventsModule,
+  DatabaseModule
   ],
   controllers: [AppController],
   providers: [AppService],
diff --git a/costly-api/src/database/database.module.ts b/costly-api/src/database/database.module.ts
index 7f44f03..d8308b4 100644
--- a/costly-api/src/database/database.module.ts
+++ b/costly-api/src/database/database.module.ts
@@ -6,26 +6,14 @@ import { GroupEventEntity } from 'src/groups/persistence/group-event.entity';
 import { ExpenseEntity } from 'src/expenses/persistence/expense.entity';
 import { ExpenseEventEntity } from 'src/expenses/persistence/expense-event.entity';
 import { GroupMemberEntity } from 'src/groups/persistence/group-member.entity';
+import { ExpenseSplitEntity } from 'src/expenses/persistence/expense-split.entity';
 
-const ENTITY = [GroupEntity, GroupEventEntity, ExpenseEntity, ExpenseEventEntity, GroupMemberEntity]
+const ENTITY = [GroupEntity, GroupEventEntity, ExpenseEntity, ExpenseEventEntity, GroupMemberEntity, ExpenseSplitEntity]
 
 @Module({
   imports: [
     
-    TypeOrmModule.forRootAsync({
-      imports: [ConfigModule],
-      inject: [ConfigService],
-      useFactory: (configService: ConfigService) => ({
-          type: 'mysql',
-          host: configService.get('DATABASE_HOST') || 'localhost',
-          port: parseInt(configService.get('DATABASE_PORT') || '0'),
-          username: configService.get('DATABASE_USER'),
-          password: configService.get('DATABASE_PASSWORD'),
-          database: configService.get('DATABASE_NAME'),
-          entities: ENTITY,
-          synchronize: (configService.get('DATABASE_SYNC')||'').toLowerCase() == 'true',
-      })
-    })
+    TypeOrmModule.forFeature(ENTITY)
   ],
   providers: [ConfigService,],
   exports: [TypeOrmModule]
diff --git a/costly-api/src/expenses/dto/create-expense.dto.ts b/costly-api/src/expenses/dto/create-expense.dto.ts
index a8c084d..3084330 100644
--- a/costly-api/src/expenses/dto/create-expense.dto.ts
+++ b/costly-api/src/expenses/dto/create-expense.dto.ts
@@ -1,5 +1,5 @@
 export class CreateExpenseDto {
   name: string;
   amount: number;
-  groupId: number;
+  groupId: string;
 }
\ No newline at end of file
diff --git a/costly-api/src/expenses/expenses.module.ts b/costly-api/src/expenses/expenses.module.ts
index 42cc44b..d4042d7 100644
--- a/costly-api/src/expenses/expenses.module.ts
+++ b/costly-api/src/expenses/expenses.module.ts
@@ -3,14 +3,10 @@ import { DatabaseModule } from 'src/database/database.module';
 import { ExpensesService } from './application/expenses/expenses.service';
 import { ExpensesController } from './api/expenses/expenses.controller';
 import { ExpensesRepository } from './persistence/expense.repository';
-import { TypeOrmModule } from '@nestjs/typeorm';
-import { ExpenseEntity } from './persistence/expense.entity';
-import { ExpenseEventEntity } from './persistence/expense-event.entity';
 
 @Module({
   imports: [
-    DatabaseModule,
-    TypeOrmModule.forFeature([ExpenseEntity, ExpenseEventEntity])
+    DatabaseModule
   ],
   providers: [ExpensesService, ExpensesRepository],
   controllers: [ExpensesController]
diff --git a/costly-api/src/expenses/persistence/expense-split.entity.ts b/costly-api/src/expenses/persistence/expense-split.entity.ts
new file mode 100644
index 0000000..679c81a
--- /dev/null
+++ b/costly-api/src/expenses/persistence/expense-split.entity.ts
@@ -0,0 +1,19 @@
+import { Column, Entity, ManyToOne, PrimaryColumn, PrimaryGeneratedColumn } from "typeorm";
+import { ExpenseEntity } from "./expense.entity";
+import { GroupMemberEntity } from "src/groups/persistence/group-member.entity";
+
+@Entity('expense_splits')
+export class ExpenseSplitEntity {
+
+  @PrimaryGeneratedColumn('uuid')
+  id: string;
+  
+  @ManyToOne(expense => ExpenseEntity, expense => expense.splits)
+  expense: ExpenseEntity;
+
+  @Column({ type: 'float'})
+  weight: number;
+
+  @ManyToOne(member => GroupMemberEntity, member => member.splits, { eager: true })
+  member: GroupMemberEntity;
+}
\ No newline at end of file
diff --git a/costly-api/src/expenses/persistence/expense.entity.ts b/costly-api/src/expenses/persistence/expense.entity.ts
index 051fe53..eec3bc6 100644
--- a/costly-api/src/expenses/persistence/expense.entity.ts
+++ b/costly-api/src/expenses/persistence/expense.entity.ts
@@ -1,5 +1,6 @@
 import { GroupEntity } from "src/groups/persistence/group.entity";
-import { Column, CreateDateColumn, Entity, OneToMany, PrimaryGeneratedColumn, UpdateDateColumn } from "typeorm";
+import { Column, CreateDateColumn, Entity, ManyToOne, OneToMany, PrimaryGeneratedColumn, UpdateDateColumn } from "typeorm";
+import { ExpenseSplitEntity } from "./expense-split.entity";
 
 @Entity('expenses')
 export class ExpenseEntity {
@@ -10,9 +11,12 @@ export class ExpenseEntity {
   @Column({ nullable: false })
   name: string;
 
-  @OneToMany(type => GroupEntity, group => group.expenses)
+  @ManyToOne(type => GroupEntity, group => group.expenses)
   group: GroupEntity;
 
+  @OneToMany(type => ExpenseSplitEntity, split => split.expense, { eager: true, cascade: true})
+  splits: ExpenseSplitEntity[];
+
   @Column()
   amount: number;
 
diff --git a/costly-api/src/group-events/api/group-events.controller.ts b/costly-api/src/group-events/api/group-events.controller.ts
index 6d01945..7b3dcf2 100644
--- a/costly-api/src/group-events/api/group-events.controller.ts
+++ b/costly-api/src/group-events/api/group-events.controller.ts
@@ -1,4 +1,4 @@
-import { Body, Controller, Get, Param, Post } from '@nestjs/common';
+import { Body, Controller, Get, Param, Post, Req, Session } from '@nestjs/common';
 import { GroupEventsService } from '../application/group-events.service';
 import { IncomingGroupEventDto } from '../dto/group-event.dto';
 
@@ -17,13 +17,13 @@ export class GroupEventsController {
   }
 
   @Get()
-  getEvents() {
-    return this.groupsEventsService.getLastTenEvents();
+  getEvents(@Session() session: Record<string, any>) {
+    return this.groupsEventsService.getLastEvents({});
   }
 
   @Get(':groupdId')
   getGroupEvents(@Param('groupId') groupId: string) {
-    return this.groupsEventsService.getLastTenEvents(groupId);
+    return this.groupsEventsService.getLastEvents({groupId});
   }
 
 }
diff --git a/costly-api/src/group-events/application/group-events.service.ts b/costly-api/src/group-events/application/group-events.service.ts
index 0dc1a45..7f62bf9 100644
--- a/costly-api/src/group-events/application/group-events.service.ts
+++ b/costly-api/src/group-events/application/group-events.service.ts
@@ -9,8 +9,9 @@ import { ExpenseEntity } from 'src/expenses/persistence/expense.entity';
 import { GroupsRepository } from 'src/groups/persistence/group.repository';
 import { InjectRepository } from '@nestjs/typeorm';
 import { IncomingEventDto } from 'src/model/dto/incoming-event.dto';
-import { IncomingGroupEventDto } from '../dto/group-event.dto';
+import { GroupEventType, GROUPEVENTTYPE, GroupRenamedPayload, IncomingGroupEventDto, TypedIncomingGroupEvent } from '../dto/group-event.dto';
 import { GroupMemberEntity } from 'src/groups/persistence/group-member.entity';
+import { ExpenseSplitEntity } from 'src/expenses/persistence/expense-split.entity';
 
 @Injectable()
 export class GroupEventsService {
@@ -57,13 +58,15 @@ export class GroupEventsService {
         // optional: nur diejenigen anwenden, die "gerade neu" sind
         // (Wenn du ganz sauber sein willst: execute() liefert affected, aber nicht IDs.
         //  Pragmatik: apply ist idempotent, dann ist doppelt anwenden ok, aber nur wenn Operationen idempotent sind!)
-        for (const ev of persisted) {
+        for (const ev of (persisted as TypedIncomingGroupEvent[])) {
           if (ev.type === 'GROUP_RENAMED') {
-            await this.ingestGroupRenameEvent(groupId, ev)
+            await this.ingestGroupRenameEvent(groupId, ev as any)
           } else if (ev.type == 'MEMBER_CREATED') {
-            await this.ingestGroupAddMemberEvent(groupId, ev);
+            await this.ingestGroupAddMemberEvent(groupId, ev as any);
           } else if (ev.type == 'MEMBER_RENAMED') {
-            await this.ingestMemberRenamedEvent(groupId, ev);
+            await this.ingestMemberRenamedEvent(groupId, ev as any);
+          } else if (ev.type == 'EXPENSE_CREATED') {
+            await this.ingestCreateExpense(groupId, ev as any)
           }
         }
   
@@ -71,11 +74,11 @@ export class GroupEventsService {
       });
   }
 
-  async ingestMemberRenamedEvent(groupId: string, ev: GroupEventEntity) {
+  async ingestMemberRenamedEvent(groupId: string, ev: TypedIncomingGroupEvent<'MEMBER_RENAMED'>) {
     if (!ev.payload.to) { throw new BadRequestException('to is required'); }
     if (!ev.payload.from) { throw new BadRequestException('from is required'); }
     try {
-      const member = await this.groupMembers.findOneByOrFail({ id: ev.payload.id + '', group: { id: groupId }});
+      const member = await this.groupMembers.findOneByOrFail({ id: ev.payload.memberId + '', group: { id: groupId }});
       member.name = ev.payload.to;
       await this.groupMembers.save(member);
       return member;
@@ -84,7 +87,7 @@ export class GroupEventsService {
     }
   }
 
-  async ingestGroupAddMemberEvent(groupId: string, ev: GroupEventEntity) {
+  async ingestGroupAddMemberEvent(groupId: string, ev: TypedIncomingGroupEvent<'MEMBER_CREATED'>) {
     const group = await this.groupsRepo.findOneByOrFail({ id: groupId });
     const member = await this.groupMembers.save(this.groupMembers.create({
       name: ev.payload.name,
@@ -92,20 +95,43 @@ export class GroupEventsService {
     }))
     return member;
   }
-  async ingestGroupRenameEvent(groupId: string, ev: GroupEventEntity) {
+  async ingestGroupRenameEvent(groupId: string, ev: TypedIncomingGroupEvent<'GROUP_RENAMED'>) {
     const group = await this.groupsRepo.findOneByOrFail({ id: groupId });
     group.name = ev.payload.to;
     const saved = await this.groupsRepo.save(group);
     return saved;
   }
 
-  getLastTenEvents(groupId?: string) {
+  getLastEvents({groupId, amount}: {groupId?: string, amount?: 10}) {
     const rep = this.dataSource.getRepository(GroupEventEntity);
     return rep.find({
       where: { groupId },
       order: { createdAt: 'DESC' },
-      take: 10
+      take: amount
 
     })
   }
+
+  async ingestCreateExpense(groupId: string, ev: GroupEventEntity) {
+    const group = await this.groupsRepo.findOneByOrFail({ id: groupId });
+    const repo = this.dataSource.getRepository(ExpenseEntity);
+    const splitRepo = this.dataSource.getRepository(ExpenseSplitEntity);
+    const dto = ev.payload as CreateExpenseDto;
+    dto.groupId = groupId;
+    const exp = repo.create(dto);
+    exp.group = group;
+    
+    const expense = await repo.save(exp)
+    for (let m of group.members) {
+      const s = await splitRepo.save(
+        splitRepo.create({
+          expense,
+          member: m,
+          weight: 1 / group.members.length
+        })
+      )
+      console.log(s)
+    }
+    return exp;
+  }
 }
diff --git a/costly-api/src/group-events/dto/group-event.dto.ts b/costly-api/src/group-events/dto/group-event.dto.ts
index 4b70e48..56e746b 100644
--- a/costly-api/src/group-events/dto/group-event.dto.ts
+++ b/costly-api/src/group-events/dto/group-event.dto.ts
@@ -12,4 +12,56 @@ export class IncomingGroupEventDto {
 
   @IsOptional() @IsISO8601()
   clientCreatedAt?: string;
-}
\ No newline at end of file
+}
+
+export type GroupEventType =
+  | 'GROUP_RENAMED'
+  | 'EXPENSE_CREATED'
+  | 'EXPENSE_AMOUNT_MODIFIED'
+  | 'MEMBER_CREATED'
+  | 'MEMBER_RENAMED';
+
+export interface GroupRenamedPayload {
+  from: string;
+  to: string;
+}
+
+export interface ExpenseCreatedPayload {
+  expenseId: string;
+  payerMemberId: string;
+  amountCents: number;
+  splits: Array<{ memberId: string; weight: number }>;
+}
+
+export interface ExpenseAmountModifiedPayload {
+  expenseId: string;
+  fromAmountCents: number;
+  toAmountCents: number;
+}
+
+export interface MemberCreatedPayload {
+  memberId: string;
+  name: string;
+}
+
+export interface MemberRenamedPayload {
+  memberId: string;
+  from: string;
+  to: string;
+}
+
+export type GroupEventPayloadByType = {
+  GROUP_RENAMED: GroupRenamedPayload;
+  EXPENSE_CREATED: ExpenseCreatedPayload;
+  EXPENSE_AMOUNT_MODIFIED: ExpenseAmountModifiedPayload;
+  MEMBER_CREATED: MemberCreatedPayload;
+  MEMBER_RENAMED: MemberRenamedPayload;
+};
+
+export type TypedIncomingGroupEvent<T extends GroupEventType = GroupEventType> = {
+  id: string;
+  type: T;
+  payload: GroupEventPayloadByType[T];
+  actorId?: string;
+  clientCreatedAt?: string;
+};
diff --git a/costly-api/src/group-events/group-events.module.ts b/costly-api/src/group-events/group-events.module.ts
index fd05966..a1ef280 100644
--- a/costly-api/src/group-events/group-events.module.ts
+++ b/costly-api/src/group-events/group-events.module.ts
@@ -9,6 +9,6 @@ import { GroupMemberEntity } from 'src/groups/persistence/group-member.entity';
 @Module({
   controllers: [GroupEventsController],
   providers: [GroupEventsService],
-  imports: [DatabaseModule, TypeOrmModule.forFeature([GroupEventsController, GroupEntity, GroupMemberEntity])]
+  imports: [DatabaseModule]
 })
 export class GroupEventsModule {}
diff --git a/costly-api/src/groups/groups.module.ts b/costly-api/src/groups/groups.module.ts
index 75c56e5..0b78285 100644
--- a/costly-api/src/groups/groups.module.ts
+++ b/costly-api/src/groups/groups.module.ts
@@ -5,9 +5,10 @@ import { GroupsController } from './api/groups.controller';
 import { GroupsService } from './application/groups.service';
 import { GroupsRepository } from './persistence/group.repository';
 import { GroupEventEntity } from './persistence/group-event.entity';
+import { DatabaseModule } from 'src/database/database.module';
 
 @Module({
-  imports: [TypeOrmModule.forFeature([GroupEntity, GroupEventEntity])],
+  imports: [DatabaseModule],
   controllers: [GroupsController],
   providers: [GroupsService, GroupsRepository]
 })
diff --git a/costly-api/src/groups/persistence/group-member.entity.ts b/costly-api/src/groups/persistence/group-member.entity.ts
index d4f31b6..70f1f3a 100644
--- a/costly-api/src/groups/persistence/group-member.entity.ts
+++ b/costly-api/src/groups/persistence/group-member.entity.ts
@@ -1,5 +1,6 @@
 import { Column, CreateDateColumn, Entity, ManyToOne, OneToMany, PrimaryGeneratedColumn, UpdateDateColumn } from "typeorm";
 import { GroupEntity } from "./group.entity";
+import { ExpenseSplitEntity } from "src/expenses/persistence/expense-split.entity";
 
 @Entity('group_members')
 export class GroupMemberEntity {
@@ -10,9 +11,14 @@ export class GroupMemberEntity {
   @Column({ nullable: false })
   name: string;
 
+  balance: number = 0;
+
   @ManyToOne(type => GroupEntity, group => group.members)
   group: GroupEntity;
 
+  @OneToMany(type => ExpenseSplitEntity, split => split.member)
+  splits: ExpenseSplitEntity[];
+
   @CreateDateColumn({ name: 'created_at' })
   createdAt: Date;
   
diff --git a/costly-api/src/groups/persistence/group.entity.ts b/costly-api/src/groups/persistence/group.entity.ts
index 205172e..dca3b9c 100644
--- a/costly-api/src/groups/persistence/group.entity.ts
+++ b/costly-api/src/groups/persistence/group.entity.ts
@@ -17,7 +17,8 @@ export class GroupEntity {
   @UpdateDateColumn({ name: 'updated_at' })
   updatedAt: Date;
 
-  expenses: ExpenseEntity[] = [];
+  @OneToMany(type => ExpenseEntity, expense => expense.group, { eager: true })
+  expenses: ExpenseEntity[];
   
   @OneToMany(type => GroupMemberEntity, member => member.group, { eager: true })
   members: GroupMemberEntity[];
diff --git a/costly-api/src/groups/persistence/group.repository.ts b/costly-api/src/groups/persistence/group.repository.ts
index 6941955..4bd9323 100644
--- a/costly-api/src/groups/persistence/group.repository.ts
+++ b/costly-api/src/groups/persistence/group.repository.ts
@@ -12,14 +12,24 @@ export class GroupsRepository {
     private readonly repo: Repository<GroupEntity>,
   ) {}
 
-  findById(id: string) {
-    return this.repo.findOne({ where: { id } });
+  async findById(id: string) {
+    const group = await this.repo.findOneOrFail({ where: { id } });
+    this.addTotals(group);
+    return group;
   }
 
   all() {
     return this.repo.find();
   }
 
+  addTotals(group: GroupEntity) {
+    for (let m of group.members) {
+      const expenses = group.expenses.filter(ex => ex.splits.some( s => s.member.id == m.id));
+      const total = expenses.reduce((a, b) => a + b.amount * ((b.splits || []).find(s => s.member.id == m.id)?.weight || 0), 0);
+      m.balance = m.balance - total;
+    }
+  }
+
 createGroup(
     dto: CreateGroupDto,
     manager?: EntityManager,
diff --git a/costly-api/src/main.ts b/costly-api/src/main.ts
index 49efa03..8878c0a 100644
--- a/costly-api/src/main.ts
+++ b/costly-api/src/main.ts
@@ -1,9 +1,20 @@
 import { NestFactory } from '@nestjs/core';
 import { AppModule } from './app.module';
 import { ValidationPipe } from '@nestjs/common';
+import * as session from 'express-session';
 
 async function bootstrap() {
   const app = await NestFactory.create(AppModule);
+
+  // somewhere in your initialization file
+  app.use(
+    session({
+      secret: 'sdfjsodjf',
+      resave: false,
+      saveUninitialized: false,
+    }),
+  );
+
   app.useGlobalPipes(
     new ValidationPipe({
       whitelist: true,            // entfernt unbekannte Felder