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Table of contents

1. restart MongoDB
2. projection in pyMongo
3. Copy one collection from one database to another
4. Remove one filed in collection
5. Rename the filed in collection
6. append an item to an array in mongoDB

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restart MongoDB

When the server crashed down, and you failed to connect to the mongoDB server after typing mongosh. Then you need to restart the mongoDB server as shown below.

sudo service mongod restart

projection in pyMongo

query = {'status':'active', 'testOrdered':testOrdered,'panel':panel,'specimen':specimen}
list_tag = list(collection_rule_extraction.find(query, projection={'_id':False,'tag':True}))
list_tag = [rule['tag'] for rule in list_tag]

You need to use True or False to specify which filed needed to be returned.

Copy one collection from one database to another

1. Install mongosh
2. In the shell using the command line to backup: mongodump -d some_database -c some_collection (e.g. mongodump -d comment -c comment_ST)
3. restore the collection to another database: mongorestore -d some_other_db -c some_or_other_collection dump/some_collection.bson (e.g. mongorestore -d report -c comment_ST dump/comment/comment_ST.bson)
4. Done!

Remove one filed in collection

db.example.update({}, {$unset: {words:1}} , {multi: true});

Add one filed in collection:

db.example.update({}, {$set: {'status':'active'}} , {multi: true});

Rename the filed in collection

go into the mongosh command line interface

db.collectionName.update({}, {
    $rename: {
        "old": "new"
    }
}, false, true);


//e.g.
db.foo.update({}, {
    $rename: {
        "name.additional": "name.last"
    }
}, false, true);

The false, true in the method above are: { upsert:false, multi:true }. You need the multi:true to update all your records.

append an item to an array in mongoDB

coll.update({'ref': ref}, {'$push': {'tags': new_tag}})