PouchDB-映射/减少对SQL group by的模仿？

Question

请考虑以下存储在pouchdb中的示例文档。

{
  "_id": "milk-20190509T010751.826-1",
  "animal": "B",
  "shift": "M",
  "ltr": 5,
  "code": 1,
  "custName": "Balvant Jat",
  "type": "milk",
  "netAmt": 200,
  "billName": "1951",
}

{
  "_id": "milk-20190509T010751.827-1",
  "animal": "B",
  "shift": "M",
  "ltr": 6,
  "code": 1,
  "custName": "Balvant Jat",
  "type": "milk",
  "netAmt": 150,
  "billName": "1951",
}

{
  "_id": "milk-20190509T010751.828-2",
  "animal": "B",
  "shift": "M",
  "ltr": 6,
  "code": 2,
  "custName": "Ashvin Jat",
  "type": "milk",
  "netAmt": 500,
  "billName": "1951",
}

我尝试了一些map / reduce函数，还使用了group:true选项，但无法获得预期的结果。我尝试了如下的地图功能

mapFunc = (doc) => {
   if(doc.type === 'milk'){
   emit(doc.billName,{code:doc.code,name:doc.name,ltr:doc.ltr,amount:doc.netAmt})
   }
}

但不知道我在哪里停下reduce函数。

我想成为如下所示的文档。

{
  "code": 1,
  "custName": "Balvant Jat",
  "totalLtr": 11 
  "totalNetAmt": 350,
}

{
  "code": 2,
  "custName": "Ashvin Jat",
  "totalLtr": 6
  "totalNetAmt": 500,
}