Mock What Prisma Returns, Not What Your API Returns

I'm building RunHop in public — a social + event platform for running races, built on NestJS. Today I finished the organization and membership e2e test suites. Along the way, I spent an embarrassing amount of time on a TypeError that came down to one wrong mock. The Crash // organization.service.ts async list ( cursor ?: string , take : number = 20 ) { const args : Prisma . OrganizationFindManyArgs = { take , where : { deletedAt : null }, orderBy : { createdAt : ' desc ' } }; if ( cursor ) { args . skip = 1 ; args . cursor = { id : cursor } } const result = await this . prisma . organization . findMany ( args ); const nextCursor = result . at ( - 1 )?. id ; return { data : result , meta : { cursor : nextCursor } }; } The unit test: it ( ' should return orgs without cursor ' , async () => { mockPrisma . organization . findMany . mockResolvedValue ( { data : [ mockOrg ], nextCursor : mockOrg . id } // <-- the problem ); const result = await service . list (); // TypeError: result.at is n