Panic on using <relation> instead of <relation>_id on groupBy

[fredcarle]

% 2023/06/02 03:58:58 http: panic serving 172.17.0.1:35934: runtime error: index out of range [0] with length 0
goroutine 81290 [running]:
net/http.(*conn).serve.func1()
	net/http/server.go:1850 +0xb8
panic({0x119c100, 0x4009f1c030})
	runtime/panic.go:890 +0x260
github.com/sourcenetwork/defradb/core.(*DocumentMapping).FirstIndexOfName(...)
	github.com/sourcenetwork/defradb/core/doc.go:191
github.com/sourcenetwork/defradb/planner/mapper.toGroupBy(...)
	github.com/sourcenetwork/defradb/planner/mapper/mapper.go:999
github.com/sourcenetwork/defradb/planner/mapper.toTargetable(0x0, 0x4009f49c00, 0x400a1362a0)
	github.com/sourcenetwork/defradb/planner/mapper/mapper.go:865 +0x3d4
github.com/sourcenetwork/defradb/planner/mapper.toSelect(0x40006381c0?, 0x0?, 0x4009f49c00, {0x0?, 0x1?})
	github.com/sourcenetwork/defradb/planner/mapper/mapper.go:99 +0x358
github.com/sourcenetwork/defradb/planner/mapper.ToSelect({0x1746c18?, 0x400a1521b0?}, {0x1754920?, 0x4009f6a0a0?}, 0x2522968?)
	github.com/sourcenetwork/defradb/planner/mapper/mapper.go:35 +0x68
github.com/sourcenetwork/defradb/planner.(*Planner).newPlan(0x400a153950, {0x10779e0?, 0x4009f49c00?})
	github.com/sourcenetwork/defradb/planner/planner.go:120 +0x27c
github.com/sourcenetwork/defradb/planner.(*Planner).newPlan(0x400a153950, {0xf60800?, 0x400a1538f0?})
	github.com/sourcenetwork/defradb/planner/planner.go:117 +0x1b4
github.com/sourcenetwork/defradb/planner.(*Planner).newPlan(0x400a153950, {0xf60900?, 0x4009f6af50?})
	github.com/sourcenetwork/defradb/planner/planner.go:106 +0x144
github.com/sourcenetwork/defradb/planner.(*Planner).makePlan(0x400a2f8c18?, {0xf60900?, 0x4009f6af50?})
	github.com/sourcenetwork/defradb/planner/planner.go:172 +0x28
github.com/sourcenetwork/defradb/planner.(*Planner).RunRequest(0x4009f6a050?, {0x1746c18, 0x400a1521b0}, 0x4009f6af50)
	github.com/sourcenetwork/defradb/planner/planner.go:474 +0x50
github.com/sourcenetwork/defradb/db.(*db).execRequest(0x4000624000, {0x1746c18?, 0x400a1521b0}, {0x40006b7980, 0x34}, {0x1754920?, 0x4009f6a0a0})
	github.com/sourcenetwork/defradb/db/request.go:53 +0x388
github.com/sourcenetwork/defradb/db.(*implicitTxnDB).ExecRequest(0x4000010110, {0x1746c18, 0x400a1521b0}, {0x40006b7980, 0x34})
	github.com/sourcenetwork/defradb/db/txn_db.go:44 +0x180
github.com/sourcenetwork/defradb/api/http.execGQLHandler({0x17432a0, 0x4009f74040}, 0x4009f48200)
	github.com/sourcenetwork/defradb/api/http/handlerfuncs.go:148 +0x5b4
github.com/sourcenetwork/defradb/api/http.(*handler).handle.func1({0x17432a0, 0x4009f74040}, 0x4009f48100)
	github.com/sourcenetwork/defradb/api/http/handler.go:86 +0x2e0
net/http.HandlerFunc.ServeHTTP(0x102f340?, {0x17432a0?, 0x4009f74040?}, 0x400a1d4005?)
	net/http/server.go:2109 +0x38
github.com/go-chi/chi/v5.(*Mux).routeHTTP(0x400007d740, {0x17432a0, 0x4009f74040}, 0x4009f48100)
	github.com/go-chi/chi/v5@v5.0.8/mux.go:444 +0x22c
net/http.HandlerFunc.ServeHTTP(0x1159d80?, {0x17432a0?, 0x4009f74040?}, 0x0?)
	net/http/server.go:2109 +0x38
github.com/sourcenetwork/defradb/api/http.loggerMiddleware.func1({0x1744d10?, 0x4000728000}, 0x4009f48100)
	github.com/sourcenetwork/defradb/api/http/logger.go:57 +0xbc
net/http.HandlerFunc.ServeHTTP(0x40008860a0?, {0x1744d10?, 0x4000728000?}, 0x4009f48100?)
	net/http/server.go:2109 +0x38
github.com/go-chi/cors.(*Cors).Handler.func1({0x1744d10, 0x4000728000}, 0x4009f48100)
	github.com/go-chi/cors@v1.2.1/cors.go:228 +0x1e4
net/http.HandlerFunc.ServeHTTP(0x1746b70?, {0x1744d10?, 0x4000728000?}, 0x225df90?)
	net/http/server.go:2109 +0x38
github.com/go-chi/chi/v5.(*Mux).ServeHTTP(0x400007d740, {0x1744d10, 0x4000728000}, 0x4009f48000)
	github.com/go-chi/chi/v5@v5.0.8/mux.go:90 +0x308
net/http.serverHandler.ServeHTTP({0x173e610?}, {0x1744d10, 0x4000728000}, 0x4009f48000)
	net/http/server.go:2947 +0x2cc
net/http.(*conn).serve(0x400058f720, {0x1746c18, 0x4000804870})
	net/http/server.go:1991 +0x544
created by net/http.(*Server).Serve
	net/http/server.go:3102 +0x43c