我必须对多个数据库进行查询。基本上,一组承诺必须按顺序完成。我在一个while循环中实现它。数据库是根据日期命名的。我还必须将请求延迟设置为400ms乘以每个诺言之间传递的循环数量(400 * count)。如果有帮助,我将cloudant用作数据库,类似于mongodb / couchdb,但具有在线实例。
因此,我实现了一个查询函数,该函数将采用开始和结束日期,并将检索范围内的所有值。如果日期相同,则将查询一个数据库,否则将查询日期范围之间每天的多个数据库。我主要在if语句中的else块遇到麻烦,如下所示。
db.predQuery = (start, end, locationCode) => {
return new Promise((resolve, reject) => {
let data = [];
const startDate = moment(start, "MM/DD/YYYY");
const endDate = moment(end, "MM/DD/YYYY");
if (startDate.diff(endDate) === 0) {
// THIS IF BLOCK WORKS GOOD
let dbName = `prediction_${String(locationCode)}_`;
dbName += startDate.format("YYYY-MM-DD");
const db = this.cloudant.use(dbName);
return db.find({selector: {_id: {'$gt': 0}}}).then((body) => {
data = body.docs;
return resolve(data);
});
} else {
// THIS BLOCK IS WHERE THE PROBLEM IS
// This is to start off the promise chain in the while loop
let chainProm = Promise.resolve(1);
let iterator = moment(startDate);
let count = 0;
// While loop for the series of promises
while (iterator.isBefore(endDate) || iterator.isSame(endDate)) {
//dbName Format: prediction_0_2019-05-28
let dbName = `prediction_${String(locationCode)}_`;
dbName += iterator.format("YYYY-MM-DD");
const db = this.cloudant.use(dbName);
count += 1;
// Set off chain of promises
chainProm = chainProm.then(() => {
setTimeout(()=>{
db.find({selector: {_id: {'$gt': 0}}}).then((body) => {
// Keep adding on to the old array
data = data.concat(body.docs);
});
},count*400);
});
// Move to the next day
iterator.add(1,'days');
}
// Once all done resolve with the array of all the documents
return resolve (data);
}
})
};
基本上,假设输出是日期范围之间所有文档的数组。现在,单个日期有效,但是当我执行一系列日期时,请求没有通过,或者我达到了极限,或者说承诺从未解决。我认为这可能不是解决此问题的最佳方法,并且可以接受任何解决方案。任何帮助是极大的赞赏。
您chainProm
没有与的外部呼叫建立联系predQuery
-该呼叫resolve
被立即呼叫。
您可能会发现使用async
/更容易,await
相反,delay-inside-loop逻辑将更容易理解:
const delay = ms => new Promise(resolve => setTimeout(resolve, ms));
db.predQuery = async (start, end, locationCode) => {
const startDate = moment(start, "MM/DD/YYYY");
const endDate = moment(end, "MM/DD/YYYY");
if (startDate.diff(endDate) === 0) {
// THIS IF BLOCK WORKS GOOD
let dbName = `prediction_${String(locationCode)}_`;
dbName += startDate.format("YYYY-MM-DD");
const db = this.cloudant.use(dbName);
const body = await db.find({selector: {_id: {'$gt': 0}}});
return body.docs;
}
const iterator = moment(startDate);
const data = [];
const testShouldIterate = () => iterator.isBefore(endDate) || iterator.isSame(endDate);
let shouldIterate = testShouldIterate();
while (shouldIterate) {
//dbName Format: prediction_0_2019-05-28
let dbName = `prediction_${String(locationCode)}_`;
dbName += iterator.format("YYYY-MM-DD");
const db = this.cloudant.use(dbName);
const body = await db.find({selector: {_id: {'$gt': 0}}});
data.push(body.docs);
// Move to the next day
iterator.add(1,'days');
shouldIterate = testShouldIterate();
// Don't delay on the final iteration:
if (!shouldIterate) {
break;
}
await delay(400);
}
// Once all done resolve with the array of all the documents
return data;
};
使用此实现,任何错误都将被发送到的调用者predQuery
(错误将导致它返回的Promise被拒绝),因此在调用时predQuery
,请务必放一个catch
之后。
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