Yeah, it&#39;s possible using [$exists][1]:

    db.things.find( { a : { $exists : false } } ); // return if a is missing

When &lt;boolean&gt; is true, $exists matches the documents that contain the field, including documents where the field value is null. If &lt;boolean&gt; is false, the query returns only the documents that do not contain the field.

  [1]: https://docs.mongodb.com/manual/reference/operator/query/exists/

If you don&#39;t care if the field is missing or `null` (or if it&#39;s never `null`) then you can use the slightly shorter _and_ safer:

    db.things.find( { a : null } ); // return if a is missing or null

It&#39;s safer because `$exists` will return `true` even if the field is null, which often is not the desired result and can lead to an NPE.

just for the reference here, **for those of you using **mongoose**** (v6) and trying to use the `$exists` to find a field that is not defined in your mongoose schema, mongoose v6 will escape it.

see here https://mongoosejs.com/docs/migrating_to_6.html#strictquery-is-removed-and-replaced-by-strict

for example:

    const userSchema = new Schema({ name: String });
    const User = mongoose.model(&#39;User&#39;, userSchema);
    
    // By default, this is equivalent to `User.find()` because Mongoose filters out `notInSchema`
    await User.find({ notInSchema: 1 });
    
    // Set `strictQuery: false` to opt in to filtering by properties that aren&#39;t in the schema
    await User.find({ notInSchema: 1 }, null, { strictQuery: false });
    // equivalent:
    await User.find({ notInSchema: 1 }).setOptions({ strictQuery: false });

I have a universal app, and on the iPad version I&#39;m using `UISplitViewController` to create an interface similar to the Mail app.

I was having trouble pushing new Detail views, so I decided to use a `UINavigationController` so I could just push and pop views as needed. However, I do not want to use the navigation view or a toolbar. But no matter what I do, I can&#39;t hide the navigation bar.

I&#39;ve tried unchecking &quot;Shows Navigation Bar&quot; in IB, and I&#39;ve also tried setting:

    [self.navigationController setNavigationBarHidden:YES];

in the `viewDidLoad`/`viewDidAppear`/`viewWillAppear`. I&#39;ve also tried it in each of the views that will be pushed. Nothing works.

Is there something I&#39;m missing here? Is it possible to have a `UINavigationController` without a toolbar or navigation bar?

UINavigationController without navigation bar?

Is there any way to create a variable in a module in Ruby that would behave similar to a class variable? What I mean by this is that it would be able to be accessed without initializing an instance of the module, but it can be changed (unlike constants in modules).

Create module variables in Ruby

How to search for documents in a collection that are missing a certain field in MongoDB?

Mongo: find items that don&#39;t have a certain field

<p>How to search for documents in a collection that are missing a certain field in MongoDB?</p>


As someone new to Mongo, I am looking for a sample MongoDB database that I can import and play with.
Something along the lines of [world][1] for mysql or [Northwind][2] for MSSQL.


Is there one? (I couldn&#39;t find any reference to one at http://www.mongodb.org nor did my googling help)


  [1]: http://dev.mysql.com/doc/world-setup/en/index.html
  [2]: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=06616212-0356-46a0-8da2-eebc53a68034&amp;displaylang=en

Is there a sample MongoDB Database along the lines of world for MySql?

when i setup mongodb in my ubuntu , i try : ./mongo it show this error :

     couldn&#39;t connect to server 127.0.0.1 shell/mongo.js


so what can i do ,

thanks

couldn&#39;t connect to server 127.0.0.1 shell/mongo.js

I have a large collection of 300 `question` objects in a database `test`. I can interact with this collection easily through MongoDB&#39;s interactive shell; however, when I try to get the collection through Mongoose in an express.js application I get an empty array.

My question is, how can I access this already existing dataset instead of recreating it in express? Here&#39;s some code:

    var mongoose = require(&#39;mongoose&#39;);
    var Schema = mongoose.Schema;
    
    mongoose.connect(&#39;mongodb://localhost/test&#39;);
    mongoose.model(&#39;question&#39;, new Schema({ url: String, text: String, id: Number }));
    
    var questions = mongoose.model(&#39;question&#39;);
    questions.find({}, function(err, data) { console.log(err, data, data.length); });

This outputs:

    null [] 0


How to access a preexisting collection with Mongoose?

    FBFriendModel.find({
	    id: 333
	}, function (err, docs) {
	    docs.remove(); //Remove all the documents that match!
	});

The above doesn&#39;t seem to work. The records are still there. 

Can someone fix?

How do I remove documents using Node.js Mongoose?

let&#39;s say I run this query in Mongoose:

```js
    Room.find({}, (err,docs) =&gt; {
    
    }).sort({date:-1}); 
```
This doesn&#39;t work!


In Mongoose, how do I sort by date? (node.js)

In my scala code:

    QueueManager.add(getObject)

where `getObject` is a method that returns an object of type `QueueObject`.

    def getObject : QueuObject = {
        val response = //some response
        return response
    }

Is there a way I can check for the response being null, while adding the QueueObject? I know I can do this:

    if (getObject != null)
        QueueManager.add(getObject)

But I do not wish to add a level of indentation. Is there an operator that does that inline?

Thanks.

How to check for null in a single statement in scala?

In my Rails app I&#39;ve run into an issue a couple times that I&#39;d like to know how other people solve:

I have certain records where a value is optional, so some records have a value and some are null for that column.

If I order by that column on some databases the nulls sort first and on some databases the nulls sort last.

For instance, I have Photos which may or may not belong to a Collection, ie there are some Photos where `collection_id=nil` and some where `collection_id=1` etc. 

If I do `Photo.order(&#39;collection_id desc)` then on SQLite I get the nulls last but on PostgreSQL I get the nulls first.

Is there a nice, standard Rails way to handle this and get consistent performance across any database?

Rails: Order with nulls last

In VB.NET, is there a way to set a `DateTime` variable to &quot;not set&quot;? And why is it possible to set a `DateTime` to `Nothing`, but *not* possible to check if it is `Nothing`? For example:

    Dim d As DateTime = Nothing
    Dim boolNotSet As Boolean = d Is Nothing 

The second statement throws this error:

&lt;!-- language: none --&gt;

    &#39;Is&#39; operator does not accept operands of type &#39;Date&#39;. Operands must be reference or
    nullable types.







Why can&#39;t I check if a &#39;DateTime&#39; is &#39;Nothing&#39;?

I want to know the difference between `nil`, `NIL` and `null`.
I&#39;ve googled around and found this:

`nil` -&gt; null pointer to Objective-C object

`NIL` -&gt; null pointer to Objective-C class

`null` -&gt; null pointer to primitive type or absence of data

But I&#39;m not able to understand the terms &quot;Objective-C object&quot; and &quot;class&quot; clearly.

Please explain this to me. Also, is there any word like `NSNull` or `NSNil` in Objective-C? If so, then please explain for what it is for.

Difference between nil, NIL and, null in Objective-C

How can I check for null values in JavaScript? I wrote the code below but it didn&#39;t work.

    if (pass == null || cpass == null || email == null || cemail == null || user == null) {      

        alert(&quot;fill all columns&quot;);
        return false;  
    
    }   

And how can I find errors in my JavaScript programs?

How do I check for null values in JavaScript?

Yes, I know this has been asked a lot.
But, it confuses me, since the results on google for this search show different methods (listed below)

    $(document).ready(function() {
        if ($(&#39;#DivID&#39;).length){
            alert(&#39;Found with Length&#39;);
        }
        
        if ($(&#39;#DivID&#39;).length &gt; 0 ) {
            alert(&#39;Found with Length bigger then Zero&#39;);
        }
        
        if ($(&#39;#DivID&#39;) != null ) {
            alert(&#39;Found with Not Null&#39;);
        }
    });

Which one of the 3 is the correct way to check if the div exists?

EDIT:
It&#39;s a pitty to see that people do not want to learn what is the better approach from the three different methods. This question is not actually on &quot;How to check if a div exists&quot; but it&#39;s about which method is better, and, if someone could explain, why it it better?

Check if a div exists with jquery

If we have a `SELECT` statement inside an `IF EXISTS`, does the execution stop as soon as it finds a record in the table? For example:


    IF EXISTS(SELECT *  FROM  table1  WHERE Name=&#39;John&#39; )
    
    return 1
    
    else
    
    return 0

If a row exists in the table with the name = John, does it stops execution and returns 1 or does it traverses through the entire table looking for more matches?

 

IF EXISTS in T-SQL

I have a tableA:
 

    ID value
     1  100
     2  101
     2  444
     3  501

Also TableB 
  

    ID Code
    1
    2

Now I want to populate col = code of table B if there exists ID = 2 in tableA. for multiple values , get max value.
else populate it with &#39;123&#39;. Now here is what I used:

    if exists (select MAX(value) from #A where id = 2)
     BEGIN
     update #B
     set code = (select MAX(value) from #A where id = 2)
     from #A
     END
    
     ELSE 
     
     update #B
     set code = 123
     from #B

I am sure there is some problem in BEGIN;END or in IF EXIST;ELSE.
Basically I want to by-pass the else part if select statement in IF-part exist and vice- versa. For example if select statement of IF=part is:
 

    (select MAX(value) from #A where id = 4)

 It should just populate 123, coz ID = 4 do not exist ! 

SQL Server: IF EXISTS ; ELSE

How do I check whether data in a query exists?

For example:

    users_query = User.query.filter_by(email=&#39;x@x.com&#39;)

How I can check whether users with that email exist?

I can check this with 

    users_query.count()

but how to check it with **exists**?

sqlalchemy exists for query

I changed the datadir of a MySQL installation and all the bases moved correctly except for one. 
I can connect and `USE` the database. `SHOW TABLES` also returns me all the tables correctly, and the files of each table exists on the MySQL data directory.

However, when I try to `SELECT` something from the table, I get an error message that the table does not exist. Yet, this does not make sense since I was able to show the same table through `SHOW TABLES` statement. 

My guess is that `SHOW TABLES` lists file existence but does not check whether a file is corrupted or not. Consequently, I can list those files but not access them.

Nevertheless, it is merely a guess. I have never seen this before. Now, I cannot restart the database for testing, but every other application that uses it is running fine.
But that&#39;s just a guess, I&#39;ve never seen this before. 

Does anyone know why this is happening?

Example:

    mysql&gt; SHOW TABLES;
    +-----------------------+
    | Tables_in_database    |
    +-----------------------+
    | TABLE_ONE             |
    | TABLE_TWO             |
    | TABLE_THREE           |
    +-----------------------+
    mysql&gt; SELECT * FROM TABLE_ONE;
    ERROR 1146 (42S02): Table &#39;database.TABLE_ONE&#39; doesn&#39;t exist



Content Type	Original Author	Original Content on Stackoverflow
Question	bcmcfc	View Question on Stackoverflow
Solution 1 - Mongodb	Andrew Orsich	View Answer on Stackoverflow
Solution 2 - Mongodb	nilskp	View Answer on Stackoverflow
Solution 3 - Mongodb	Normal	View Answer on Stackoverflow

Mongo: find items that don't have a certain field

Mongodb Problem Overview

Mongodb Solutions

Solution 1 - Mongodb

Solution 2 - Mongodb

Solution 3 - Mongodb

Create module variables in Ruby

UINavigationController without navigation bar?

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