blocks|key|172323|text|本机驱动程序的Db对象的collectionNames方法接受一个可选的集合名称筛选器作为第一个参数，用于检查集合是否存在：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|172324|db.collectionNames(collName,+function(err,+names)+{
++++console.log('Exists:+',+names.length+>+0);
});|code-block|syntax|javascript|172325|在2.x版本的MongoDB本机驱动程序中，collectionNames已被listCollections所取代，后者接受筛选器并返回游标，因此您可以这样做：|172326|db.listCollections({name:+collName})
++++.next(function(err,+collinfo)+{
++++++++if+(collinfo)+{
++++++++++++//+The+collection+exists
++++++++}
++++});|172327|entityMap|0|LINK|mutability|MUTABLE|url|http://mongodb.github.io/node-mongodb-native/api-generated/db.html#collectionnames|1|http://mongodb.github.io/node-mongodb-native/2.0/api/Db.html#listCollections^0|7|2|C|F|C|F|0|0|0|M|F|13|F|13|F|1|0|0^^$0|@$1|2|3|4|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@$9|13|A|14|1|15]]|E|$]]|$1|F|3|G|5|H|7|16|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|17|8|@$9|18|A|19|B|C]|$9|1A|A|1B|B|C]]|D|@$9|1C|A|1D|1|1E]]|E|$]]|$1|M|3|N|5|H|7|1F|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|1G|8|@]|D|@]|E|$]]]|P|$Q|$5|R|S|T|E|$U|V]]|W|$5|R|S|T|E|$U|X]]]]

The <a href="http://mongodb.github.io/node-mongodb-native/api-generated/db.html#collectionnames" rel="noreferrer"><code>collectionNames</code></a> method of the native driver's <code>Db</code> object accepts an optional collection name filter as a first parameter to let you check the existence of a collection:

<pre><code>db.collectionNames(collName, function(err, names) {
 console.log('Exists: ', names.length &gt; 0);
});
</code></pre>

In the 2.x version of the MongoDB native driver, <code>collectionNames</code> has been replaced by <a href="http://mongodb.github.io/node-mongodb-native/2.0/api/Db.html#listCollections" rel="noreferrer"><code>listCollections</code></a> which accepts a filter and returns a cursor so you would do this as:

<pre><code>db.listCollections({name: collName})
 .next(function(err, collinfo) {
 if (collinfo) {
 // The collection exists
 }
 });
</code></pre>

blocks|key|172372|text|这个问题指的是本机驱动程序，但我在这里搜索如何在pymongo中做到这一点。通常pymongo的api与JS+api相同，但在这种情况下，collection_names没有集合名称的参数(如JohnnyHK的answer)，而是第一个参数是布尔值(是否包含系统集合)。由于字符串的计算结果为True，因此这可能会令人困惑。因此，我希望这对未来的读者有所帮助：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|172373|import+pymongo

cl+=+pymongo.MongoClient()
db+=+cl['my-db']
if+'my-col'+in+db.collection_names(False):
+++...|code-block|syntax|javascript|172374|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/a/28921019/165753^0|O|7|14|7|1X|G|2O|8|42|4|2X|6|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]|$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@$9|13|A|14|1|15]]|E|$]]|$1|F|3|G|5|H|7|16|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|17|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

The question refers to the native driver, but I got here searching how to do this in <code>pymongo</code>. Usually <code>pymongo</code>'s api is identical to the JS api, but in this case <code>collection_names</code> doesn't have an argument for the collection's name (as in <code>JohnnyHK</code>'s <a href="https://stackoverflow.com/a/28921019/165753">answer</a>), but rather the first argument is a boolean (whether to include system collections). Since a string evaluates to <code>True</code>, this can be confusing. So I hope this helps future readers:

<pre><code>import pymongo

cl = pymongo.MongoClient()
db = cl['my-db']
if 'my-col' in db.collection_names(False):
 ...
</code></pre>

blocks|key|4678245|text|现在，在Node.js本机驱动程序中有一个listCollections方法。它返回当前数据库中所有集合的信息。您可以使用它来检查给定的集合是否存在：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|4678246|collectionExists+=+function(name,+cb)+{
++mongoDb.listCollections().toArray(function(err,+collections)+{
++++if+(err)+return+cb(err);

++++cb(null,+collections.some(function(coll)+{
++++++return+coll.name+==+name;
++++}));
++});
}|code-block|syntax|javascript|4678247|entityMap|0|LINK|mutability|MUTABLE|url|http://mongodb.github.io/node-mongodb-native/2.0/api/Db.html#listCollections^0|L|F|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

There is now a <a href="http://mongodb.github.io/node-mongodb-native/2.0/api/Db.html#listCollections" rel="noreferrer">listCollections</a> method in Node.js native driver. It returns information on all collections in current database. You can use it to check if a given collection is there:

<pre><code>collectionExists = function(name, cb) {
 mongoDb.listCollections().toArray(function(err, collections) {
 if (err) return cb(err);

 cb(null, collections.some(function(coll) {
 return coll.name == name;
 }));
 });
}
</code></pre>

blocks|key|172453|text|从MongoDB+3.0开始，您可以简单地运行：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|172454|db.getCollectionNames()|code-block|syntax|javascript|172455|返回一个数组，其中包含当前数据库中所有集合的名称：|172456|[+"employees",+"products",+"mylogs"]|172457|检查Mongo+DB+Documentation，或者如果需要关于每个集合的更多信息，也可以使用db.getCollectionInfos()|offset|length|172458|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.mongodb.com/manual/reference/method/db.getCollectionNames/#db-getcollectionnames^0|0|0|0|0|2|M|0|0^^$0|@$1|2|3|4|5|6|7|W|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|X|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Y|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|Z|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|10|8|@]|9|@$M|11|N|12|1|13]]|A|$]]|$1|O|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|P|$Q|$5|R|S|T|A|$U|V]]]]

Since MongoDB 3.0 you can simply run:

<pre><code>db.getCollectionNames()
</code></pre>

which returns an array with the names of all the collections on current database:

<pre><code>[ "employees", "products", "mylogs"]
</code></pre>

check <a href="https://docs.mongodb.com/manual/reference/method/db.getCollectionNames/#db-getcollectionnames" rel="noreferrer">Mongo DB Documentation</a>, or you could also use db.getCollectionInfos() if you need more info about each collection

blocks|key|172495|text|如果您使用mongodb+3.1.10。这是如何检查集合是否存在的方法。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|172496|MongoClient.connect(url,+{+useNewUrlParser:+true+},+function(err,+client)+{
++if+(err)+throw+err;

++var+dbo+=+client.db("dbname");
++dbo.listCollections().toArray(function(err,+items){
++++if+(err)+throw+err;

++++console.log(items);+
++++if+(items.length+==+0)
++++++++console.log("No+collections+in+database")++
++});+
});|code-block|syntax|javascript|172497|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you use mongodb 3.1.10.
This is how to check if collections exist.

<pre><code>MongoClient.connect(url, { useNewUrlParser: true }, function(err, client) {
 if (err) throw err;

 var dbo = client.db("dbname");
 dbo.listCollections().toArray(function(err, items){
 if (err) throw err;

 console.log(items); 
 if (items.length == 0)
 console.log("No collections in database") 
 }); 
});
</code></pre>

blocks|key|4678399|text|适用于3.6.*版本的更新答案。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4678400|/**
+*+Checks+if+a+collection+exists+in+a+mongo+database.
+*+
+*+@param+db+a+mongo+db+object.++eg.
+*++++client+=+await+MongoClient.connect(uri);
+*++++db+=+client.db();
+*+@param+collectionName+the+name+of+the+collection+to+search+for
+*+@returns+{Promise<boolean>}
+*/
async+function+doesCollectionExistInDb(db,+collectionName)+{
++const+collections+=+await+db.collections();
++return+collections.some(
++++++(collection)+=>+collection.collectionName+===+collectionName
++);
}

...

if+(await+doesCollectionExistInDb(db,+'products'))+{
+++//+Do+something,+such+as+create+a+new+collection
}|code-block|syntax|javascript|4678401|collection.collectionName是文档化集合api的一部分，可以在这里找到：http://mongodb.github.io/node-mongodb-native/3.6/api/Collection.html#collectionName。|offset|length|style|CODE|4678402|entityMap|0|LINK|mutability|MUTABLE|url|http://mongodb.github.io/node-mongodb-native/3.6/api/Collection.html#collectionName^0|0|0|0|P|1B|2B|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@$I|X|J|Y|K|L]]|9|@$I|Z|J|10|1|11]]|A|$]]|$1|M|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

An updated answer which works with 3.6.* releases.
<pre class="lang-js prettyprint-override"><code>/**
 * Checks if a collection exists in a mongo database.
 * 
 * @param db a mongo db object. eg.
 * client = await MongoClient.connect(uri);
 * db = client.db();
 * @param collectionName the name of the collection to search for
 * @returns {Promise&lt;boolean&gt;}
 */
async function doesCollectionExistInDb(db, collectionName) {
 const collections = await db.collections();
 return collections.some(
 (collection) =&gt; collection.collectionName === collectionName
 );
}

...

if (await doesCollectionExistInDb(db, 'products')) {
 // Do something, such as create a new collection
}
</code></pre>
The <code>collection.collectionName</code> is part of the documented collection api as can be found here: <a href="http://mongodb.github.io/node-mongodb-native/3.6/api/Collection.html#collectionName" rel="nofollow noreferrer">http://mongodb.github.io/node-mongodb-native/3.6/api/Collection.html#collectionName</a>

blocks|key|172573|text|对于带有mongodb库(v3.6.3)的nodejs，这是我让它工作的唯一方法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|172574|const+collectionName+=+'products'
const+exists+=+(await+(await+db.listCollections().toArray()).findIndex((item)+=>+item.name+===+collectionName)+!==+-1)
console.log(exists)|code-block|syntax|javascript|172575|希望这能帮助别人|172576|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

For nodejs with mongodb library (v 3.6.3) that's the only way I got it working:
<pre><code>const collectionName = 'products'
const exists = (await (await db.listCollections().toArray()).findIndex((item) =&gt; item.name === collectionName) !== -1)
console.log(exists)
</code></pre>
Hope that helps others

blocks|key|4678457|text|/*+set+database+*/
let+db++++++++++=+client.db(+'crud'+)

/*+set+collection+*/
let+collection++=+db.collection(+'categories'+)

/*+set+query+*/
collection.find(+{}+).toArray(+(+err,+result+)+=>+{

if+(+result.length+>+0+)
{
++++console.log("Exist");
}
else
{
++++console.log("Not+Exist");

++++//+create+collection
}|type|code-block|depth|inlineStyleRanges|entityRanges|data|syntax|javascript|4678458|}|unstyled|4678459|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$B|C]]|$1|D|3|E|5|F|7|J|8|@]|9|@]|A|$]]|$1|G|3|-4|5|F|7|K|8|@]|9|@]|A|$]]]|H|$]]

<pre><code>/* set database */
let db = client.db( 'crud' )

/* set collection */
let collection = db.collection( 'categories' )

/* set query */
collection.find( {} ).toArray( ( err, result ) =&gt; {

if ( result.length &gt; 0 )
{
 console.log(&quot;Exist&quot;);
}
else
{
 console.log(&quot;Not Exist&quot;);

 // create collection
}
</code></pre>
}

blocks|key|172587|text|实际上，这对我很有效|type|unstyled|depth|inlineStyleRanges|entityRanges|data|172588|++await+db.createCollection(name,+function+(err,+res)+{
++++if+(err)+{
++++++++//console.log(err);
++++++++if+(err.codeName+=="NamespaceExists")+{
++++++++++++console.log("Already+Exists+Collection++:+"+%2B+name+%2B+"");
++++++++++++return;
++++++++}
++++}
++++console.log("Collection+created!+:+"%2Bname%2B"");

});|code-block|syntax|javascript|172589|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

actually, this works for me
<pre><code> await db.createCollection(name, function (err, res) {
 if (err) {
 //console.log(err);
 if (err.codeName ==&quot;NamespaceExists&quot;) {
 console.log(&quot;Already Exists Collection : &quot; + name + &quot;&quot;);
 return;
 }
 }
 console.log(&quot;Collection created! : &quot;+name+&quot;&quot;);

});
</code></pre>

blocks|key|172624|text|异步TypeScript函数：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|172625|/**
+*+Predicate+function+that+checks+if+a+collection+exists+in+a+given+MongoDB+database
+*
+*+@param+{Db}+db+Mongo+database+instance
+*+@param+{string}+collectionName+Name+of+collection
+*
+*+@returns+{boolean}+true+if+collection+exists,+false+otherwise
+*/
export+const+doesCollectionExist+=+async+(db:+Db,+collectionName:+string):+Promise<boolean>+=>+{
++const+cursor+=+db.listCollections({+name:+collectionName+})
++const+result+=+await+cursor.hasNext()
++await+cursor.close()

++return+result
}|code-block|syntax|javascript|172626|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

An async TypeScript function:
<pre><code>/**
 * Predicate function that checks if a collection exists in a given MongoDB database
 *
 * @param {Db} db Mongo database instance
 * @param {string} collectionName Name of collection
 *
 * @returns {boolean} true if collection exists, false otherwise
 */
export const doesCollectionExist = async (db: Db, collectionName: string): Promise&lt;boolean&gt; =&gt; {
 const cursor = db.listCollections({ name: collectionName })
 const result = await cursor.hasNext()
 await cursor.close()

 return result
}
</code></pre>

I need to check if a collection exists on a certain database and create it if it doesn't. I know that 

<pre><code>db.createCollection(collName, {strict:true}, function(error, collection))
</code></pre>

checks for existance of collection <code>collName</code> before creating it and sets <code>error</code> object. but I need an independent function to check that.

How to check if a collection exists in Mongodb native nodejs driver?

TypeScript

数据库

MongoDB

我需要检查某个数据库上是否存在一个集合，如果不存在，就创建它。db.createCollection(collName, {strict:true}, function(error, collection))在创建集合collName之前检查它是否存在，并设置error对象。但是我需要一个独立的函数来检查它。

问如何检查Mongodb原生nodejs驱动中是否存在集合？
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问如何检查Mongodb原生nodejs驱动中是否存在集合？
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