Mongodb查找不适用于Objectid

Question

Here is my code and I'm getting null value for the doc. But in the findOne({'webpageid':docs[i]._id} instead of docs[i]._id if I pass the webpageid it works. And I verified docs has the required webpage data

 // get all the records from the webpages table var collection = db.get('webpages'); collection.find({}, function(e,docs){ // iterate over the webpage list for (var i = 0; i < docs.length; i++) { // check if results table has any record for this webpage db.get('results').findOne({'webpageid':docs[i]._id}, function(err, doc) { if(err) { return; } console.log(doc); }); }; }); 

Below is the content in the 'webpages' table

 { "_id" : ObjectId("549608e16ecb16dc3c4880e6"), "name" : null, "url" : "http://www.google.com", "texttoverify" : null, "interval" : null, "websiteid" : null } { "_id" : ObjectId("549609986ecb16dc3c4880e7"), "name" : null, "url" : "http://www.google.com", "texttoverify" : null, "interval" : null, "websiteid" : null } { "_id" : ObjectId("54960a916ecb16dc3c4880e8"), "name" : "xyz", "url" : "http://www.google.com", "texttoverify" : "hello", "interval" : "00:15:00.0000000", "websiteid" : "02D7BE81-4E01-4347-BAE5-BA9A2D7EE11E" } 

Here is my content inside 'results' table

 { "_id" : ObjectId("54985a1e69af082f9c477574"), "webpageid" : "549608e16ecb16dc3c4880e6" } { "_id" : ObjectId("54986f4e69af082f9c477575"), "name" : "name" } { "_id" : ObjectId("5498d10c69af082f9c477576"), "name" : "name", "webpageid" : "\\"54960a916ecb16dc3c4880e8" } { "_id" : ObjectId("5498d1f969af082f9c477577"), "name" : "name", "webpageid" : "54960a916ecb16dc3c4880e8" } 

Thanks for looking into it and appreciate your help.

Answer 1

Your webpageids are strings while the _ids that you are searching with are ObjectIds. Your find does not match any results because there are no documents in the results table which have ObjectId values for the "webpageid" element.

There are two solutions as I see it.

1. You could store ObjectIds instead of strings for the webpageid element in the results collection. The implementation of this is of course based on how those documents get into the collection.

2. You could create a string variable from the returned ObjectId within the loop to compare instead. For example,

    ... for(var i = 0; i < docs.length; i++) { var docId = docs[i]._id.toString(); // create a string db.get('results').findOne({'webpageid':docId}, function(err, doc) ... 

If you go with the second option, you may need to look into why there is a leading quote at the beginning of the webpageid value for the second document in the results collection.

"webpageid" : "\"54960a916ecb16dc3c4880e8"

Lastly, I don't know much about your requirements, but you may want to rethink MongoDB as a solution. It appears that you are creating something like a JOIN which is something MongoDB is not designed to handle very well.