Got it:

    db.post.find({&#39;author.$id&#39;: &#39;foo&#39;})

This ``db.post.find(&#39;author.$id&#39;: &#39;foo&#39;)`` has missing the ``{}``, so the correct sentence is:

    db.post.find({&#39;author.$id&#39;: &#39;foo&#39;})

Also this can be achieved with:

    db.post.find({&#39;author&#39;: DBRef(&quot;user&quot;, ObjectId(&#39;foo&#39;))})

But is more compact and practical the first way.


You can use the .$id reference but it will ignore any indexes on those fields. 
I would suggest ignoring that method unless you are querying it directly via the terminal or want to look up something quickly. In using large collections you will want to index the field and query it using the below method. 


If you want to use an index query using the following:

    db.post.find(&#39;author&#39; : { &quot;$ref&quot; : &#39;user&#39;, &quot;$id&quot; : &#39;foo&#39; , &quot;$db&quot; :&#39;database_name&#39; })

If foo is an object id

    db.post.find(&#39;author&#39; : { &quot;$ref&quot; : &#39;user&#39;, &quot;$id&quot; : ObjectId(&#39;foo&#39;) , &quot;$db&quot; :&#39;database_name&#39; })

You can create an index on author by

    db.post.ensureIndex( {&#39;author&#39; : 1 } );






For anyone looking for a Java solution to this then if you are using [mongojack][1] its really easy:

    collection.find(DBQuery.is(&quot;user&quot;, new DBRef(user.getId(), User.class)));

Where collection is a JacksonDBCollection.


  [1]: http://mongojack.org

In mongoengine you should just use the instance of the referenced object. It should have the ID set.
Suppose the author is the Author document instance. So using this:

    Post.objects(author__eq=author)

you can go through all posts of this author.
Post.author should be defined as ReferenceField

Using Mongo 2.4.1 version

This is how you do it on command line for OLA collection where `@DBRef dbrefName`

`db.OLA.find({&quot;dbrefName.someFieldValue&quot; : &quot;Personal&quot;});`

Exact query

`db.OLA.find({&quot;dbrefName.$id&quot; : ObjectId(&quot;1234&quot;)});`

I&#39;m using Oracle&#39;s `ojdbc5.jar` and noticed that it includes two JDBC driver classes.

What is the difference between `oracle.jdbc.driver.OracleDriver` vs. `oracle.jdbc.OracleDriver`?

Which one should I use in my Java project?

Difference between Oracle jdbc driver classes?

When I run Ruby commands like `gem -v` I get this error:

&gt; /Users/kristoffer/.rvm/rubies/ruby-1.9.2-p180/bin/gem:4:
&gt; warning: Insecure world writable dir
&gt; /Users/kristoffer in PATH, mode 040777 

&gt;1.6.2

First of all I don&#39;t understand what this means. /Users/kristoffer is not in my path according to `echo $PATH`. The result of `echo $PATH` is:

&gt; /Users/kristoffer/.rvm/gems/ruby-1.9.2-p180/bin:/Users/kristoffer/.rvm/gems/ruby-1.9.2-p180@global/bin:/Users/kristoffer/.rvm/rubies/ruby-1.9.2-p180/bin:/Users/kristoffer/.rvm/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin

As you can see, the PATH is pretty clean. Just the default path + what RVM added. 

I&#39;ve seen the other posts similar to this where the recommended way to solve the issue is to run `chmod go-w path/to/folder`

However, I&#39;m pretty sure that it&#39;s a bad idea to make my Home folder non-writeable, right? I&#39;ve repaired permissions using Disk Utility and it didn&#39;t find anything wrong with the permissions on my Home folder.

Any idea of what the problem is and how I can fix it?

Insecure world writable dir /Users/username in PATH, mode 040777 when running Ruby commands

suppose I have the following datastructure:

    var user = {_id: &#39;foo&#39;, age: 35};
    var post = {_id: &#39;...&#39;, author: {$ref: user, $id: &#39;foo&#39;},...};

How can I query all posts which references user[foo]? I tried the following but not work:

    db.post.find(&#39;author._id&#39;: &#39;foo&#39;);
    var u = db.user.find({_id: &#39;foo&#39;});
    db.post.find(&#39;author&#39;: u);

neither can I find the answer from the official document and google!

Anyone has any idea?

How to query mongodb with DBRef

suppose I have the following datastructure:
<pre><code class="hljs language-ini">var user = {_id: 'foo', age: 35};
var post = {_id: '...', author: {$ref: user, $id: 'foo'},...};
</code></pre>
How can I query all posts which references user[foo]? I tried the following but not work:
<pre><code class="hljs language-lua">db.post.find('author._id': 'foo');
var u = db.user.find({_id: 'foo'});
db.post.find('author': u);
</code></pre>
neither can I find the answer from the official document and google!
Anyone has any idea?

I have read that .NET supports return of references, but C# doesn&#39;t. Is there a special reason? Why I can&#39;t do something like:

    static ref int Max(ref int x, ref int y) 
    { 
      if (x &gt; y) 
        return ref x; 
      else 
        return ref y; 
    } 

Why doesn&#39;t C# support the return of references?

Please have a look at the code below (excerpt from a C# book):

    
    public class MyClass 
    {
        public int val;
    }
    public struct myStruct 
    {
        public int val;
    }
    public class Program 
    {
        private static void Main(string[] args) 
        {
            MyClass objectA = new MyClass();
            MyClass objectB = objectA;

            objectA.val = 10;
            objectB.val = 20;

            myStruct structA = new myStruct();
            myStruct structB = structA;

            structA.val = 30;
            structB.val = 40;

            Console.WriteLine(&quot;objectA.val = {0}&quot;, objectA.val);
            Console.WriteLine(&quot;objectB.val = {0}&quot;, objectB.val);
            Console.WriteLine(&quot;structA.val = {0}&quot;, structA.val);
            Console.WriteLine(&quot;structB.val = {0}&quot;, structB.val);

            Console.ReadKey();
        }
    }
    

I understands it produces the output below:

    objectA.val = 20
    objectB.val = 20
    structA.val = 30
    structB.val = 40

The last two lines of the output I have no problem with, but the first two tell me that `objectA` and `objectB` are pointing to the same memory block (since in C#, objects are reference types).

The question is how do make `objectB`, a copy of `objectA` so that it points to a different area in memory. I understand that trying to assign their members may not work since those members may be references, too. So how do I go about making `objectB` a completely different entity from `objectA`? 

Creating a copy of an object in C#

I&#39;m looking for some good comprehensive reading material on when JavaScript passes something by value and when by reference and when modifying a passed item affects the value outside a function and when not.  I&#39;m also interested in when assigning to another variable is by reference vs. by value and whether that follows any different rules than passing as a function parameter.

I&#39;ve done a lot of searching and find lots of specific examples (many of them here on SO) from which I can start to piece together pieces of the real rules, but I haven&#39;t yet found a single, well written document that describes it all.

Also, are there ways in the language to control whether something is passed by reference or by value?

Here are some of the types of questions I want to understand.  These are just examples - I&#39;m actually looking to understand the rules the language goes by, not just the answers to specific examples.  But, here are some examples:

    function f(a,b,c) {
       a = 3;
       b.push(&quot;foo&quot;);
       c.first = false;
    }
    
    var x = 4;
    var y = [&quot;eeny&quot;, &quot;miny&quot;, &quot;mo&quot;];
    var z = {first: true};
    f(x,y,z);

When are the contents of x, y and z changed outside the scope of f for all the different types?

    function f() {
        var a = [&quot;1&quot;, &quot;2&quot;, &quot;3&quot;];
        var b = a[1];
        a[1] = &quot;4&quot;;
        // what is the value of b now for all possible data types that the array in &quot;a&quot; might hold?
    }
    
    function f() {
        var a = [{yellow: &quot;blue&quot;}, {red: &quot;cyan&quot;}, {green: &quot;magenta&quot;}];
        var b = a[1];
        a[1].red = &quot;tan&quot;;
        // what is the value of b now and why?
        b.red = &quot;black&quot;;
        // did the value of a[1].red change when I assigned to b.red?
    }

If I want to make a fully independent copy of an object (no references whatsoever), what&#39;s the best practice way to do that?

JavaScript by reference vs. by value

I understand the syntax and general semantics of pointers versus references, but how should I decide when it is more-or-less appropriate to use references or pointers in an API?

Naturally some situations need one or the other (`operator++` needs a reference argument), but in general I&#39;m finding I prefer to use pointers (and const pointers) as the syntax is clear that the variables are being passed destructively.

E.g. in the following code:

    void add_one(int&amp; n) { n += 1; }
    void add_one(int* const n) { *n += 1; }
    int main() {
      int a = 0;
      add_one(a); // Not clear that a may be modified
      add_one(&amp;a); // &#39;a&#39; is clearly being passed destructively
    }

With the pointer, it&#39;s always (more) obvious what&#39;s going on, so for APIs and the like where clarity is a big concern are pointers not more appropriate than references? Does that mean references should only be used when necessary (e.g. `operator++`)? Are there any performance concerns with one or the other?

EDIT (OUTDATED):

Besides allowing NULL values and dealing with raw arrays, it seems the choice comes down to personal preference. I&#39;ve accepted the answer below that references [Google&#39;s C++ Style Guide][1], as they present the view that &quot;References can be confusing, as they have value syntax but pointer semantics.&quot;.

Due to the additional work required to sanitise pointer arguments that should not be NULL (e.g. `add_one(0)` will call the pointer version and break during runtime), it makes sense from a maintainability perspective to use references where an object MUST be present, though it is a shame to lose the syntactic clarity.

  [1]: https://google.github.io/styleguide/cppguide.html#Reference_Arguments


When to use references vs. pointers

Please take a look at the following simple code:

    class Foo
    {
    public:
      Foo(){}
      ~Foo(){}
    
      Foo(const Foo&amp;){}
      Foo&amp; operator=(const Foo&amp;) { return *this; }
    };
    
    static Foo g_temp;
    const Foo&amp; GetFoo() { return g_temp; }

I tried to use `auto` like this:

    auto my_foo = GetFoo();

I expected that `my_foo` will be a constant reference to `Foo`, which is the return type of the function. However, the type of `auto` is `Foo`, not the reference. Furthermore, `my_foo` is created by copying `g_temp`. This behavior isn&#39;t that obvious to me.

In order to get the reference to `Foo`, I needed to write like this:

    const auto&amp; my_foo2 = GetFoo();
          auto&amp; my_foo3 = GetFoo();



**Question**: Why does `auto` deduce the return type of `GetFoo` as an object, not a reference?

C++11 auto: what if it gets a constant reference?

is there any easy way to change the database from mysql to mongoDB ?

or better any one suggest me good tutorial do it 

converting database from mysql to mongoDb

![enter image description here][1]


  [1]: http://i.stack.imgur.com/6ejPD.png

How do I design a scheme such this in MongoDB? I think there are no foreign keys!

Foreign keys in mongo?

So this is Day 3 of learning Mongo Db.  I&#39;m coming from the MySql universe...

A lot of times when I need to write a query for a MySql table I&#39;m unfamiliar with, I would use the &quot;desc&quot; command - basically telling me what fields I should include in my query.

How would I do that for a Mongo db?  I know, I know...I&#39;m searching for a schema in a schema-less database. =)  But how else would users know what fields to use in their queries? 

Am I going at this the wrong way?  Obviously I&#39;m trying to use a MySql way of doing things in a Mongo db.  What&#39;s the Mongo way? 

How do I describe a collection in Mongo?

I&#39;m using MongoDB via Mongoid integration, as well as ActiveRecord in a project.  I would like to generate migrations for active record, and Mongoid is the default when I run.

    rails g migration

Any ideas how to specify AR as my default generator for migrations, models, etc?

Thanks!

Using Active Record generators after Mongoid installation?

I would like to know if there&#39;re a command to drop every databases from my MongoDB?

I know if I want to drop only one datatable, I just need to type the name of the database like the code below but I dont want to have to specify it.

    mongo DB_NAME --eval &#39;db.dropDatabase();&#39;

Content Type	Original Author	Original Content on Stackoverflow
Question	Gelin Luo	View Question on Stackoverflow
Solution 1 - Reference	Gelin Luo	View Answer on Stackoverflow
Solution 2 - Reference	Mariano Ruiz	View Answer on Stackoverflow
Solution 3 - Reference	RedPhoenix	View Answer on Stackoverflow
Solution 4 - Reference	dannrob	View Answer on Stackoverflow
Solution 5 - Reference	Kostanos	View Answer on Stackoverflow
Solution 6 - Reference	java_dude	View Answer on Stackoverflow

How to query mongodb with DBRef

Reference Problem Overview

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Insecure world writable dir /Users/username in PATH, mode 040777 when running Ruby commands

Difference between Oracle jdbc driver classes?

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