The uniqueness constraint for _id is per collection, so yes - one and the same ID can occur once per Collection.

It&#39;s however very unlikely, if not impossible, for the same ID to be generated twice. So in order for this to happen you would have to manually insert duplicate IDs.

How can i cast to a derived class? The below approaches all give the following error:

&gt; Cannot convert from BaseType to DerivedType. No constructor could take
&gt; the source type, or constructor overload resolution was ambiguous.

    BaseType m_baseType;
    
    DerivedType m_derivedType = m_baseType; // gives same error
    
    DerivedType m_derivedType = (DerivedType)m_baseType; // gives same error
    
    DerivedType * m_derivedType = (DerivedType*) &amp; m_baseType; // gives same error

  

C++ cast to derived class

In my backbone.js app, there is a `Trips collection` that holds `Trip models`, which is working with `LocalStorage`. I am able to call `Trips.create(form_attributes)` to create and save a trip to the `Todos store`.

When I first load my app, I call `Trips.fetch({ success: trips_fetch_success })`, and `trips_fetch_success` receives a response that shows the `Trip models` that the `Trips collection` holds.

I have tried to bind `refresh` and `change` events to the `Trips collection`, but these events are not being caught, making me believe I have the wrong idea about which events `Trips.fetch` triggers.

My question: which events should `Trips.fetch` trigger? And are the events triggered on the collection or on each of the individual `Trip models`? 




What events are triggered when calling fetch() on a Backbone.js collection?

Can Mongo IDs have the same value in different collections in the same database?

MongoDB :: are Mongo IDs unique across collections?

<p>Can Mongo IDs have the same value in different collections in the same database?</p>


What is the maximum number of parameters passed to  $in query in MongoDB?

What is the maximum number of parameters passed to $in query in MongoDB?

I am using mongodb now.

I have a blogpost collection, and it has a `tags` field which is an array, e.g.

    blogpost1.tags = [&#39;tag1&#39;, &#39;tag2&#39;, &#39;tag3&#39;, &#39;tag4&#39;, &#39;tag5&#39;]
    blogpost2.tags = [&#39;tag2&#39;, &#39;tag3&#39;]
    blogpost3.tags = [&#39;tag2&#39;, &#39;tag3&#39;, &#39;tag4&#39;, &#39;tag5&#39;]
    blogpost4.tags = [&#39;tag1&#39;, &#39;tag4&#39;, &#39;tag5&#39;]

How can I do these search

1. contains `tag1`
2. contains `[&#39;tag1&#39;,&#39;tag2&#39;]`
3. contains any of `[&#39;tag3&#39;, &#39;tag4&#39;]`

How to check if an array field contains a unique value or another array in MongoDB?

In previous versions of Mongoose (for node.js) there was an option to use it without defining a schema

    var collection = mongoose.noSchema(db, &quot;User&quot;);

But in the current version the &quot;noSchema&quot; function has been removed. My schemas are likely to change often and really don&#39;t fit in with a defined schema so is there a new way to use schema-less models in mongoose?





How do you use Mongoose without defining a schema?

I&#39;m new to MongoDB--coming from a relational database background. I want to design a question structure with some comments, but I don&#39;t know which relationship to use for comments: `embed` or `reference`?

A question with some comments, like [stackoverflow](https://stackoverflow.com/), would have a structure like this:

    Question
        title = &#39;aaa&#39;
        content = bbb&#39;
        comments = ???

At first, I want to use embeded comments (I think `embed` is recommended in MongoDB), like this:

    Question
        title = &#39;aaa&#39;
        content = &#39;bbb&#39;
        comments = [ { content = &#39;xxx&#39;, createdAt = &#39;yyy&#39;}, 
                     { content = &#39;xxx&#39;, createdAt = &#39;yyy&#39;}, 
                     { content = &#39;xxx&#39;, createdAt = &#39;yyy&#39;} ]

It clear, but I&#39;m worried about this case: **If I want to edit a specified comment, how do I get its content and its question?** There is no `_id` to let me find one, nor `question_ref` to let me find its question. (I&#39;m so newbie, that I don&#39;t know if there&#39;s any way to do this without `_id` and `question_ref`.)

Do I have to use `ref` not `embed`? Then I have to create a new collection for comments?

MongoDB relationships: embed or reference?

What I want is not a comparison between Redis and MongoDB. I know they are different; the performance and the API is totally different.

Redis is very fast, but the API is very &#39;atomic&#39;. MongoDB will eat more resources, but the API is very very easy to use, and I am very happy with it.

They&#39;re both awesome, and I want to use Redis in deployment as much as I can, but it is hard to code. I want to use MongoDB in development as much as I can, but it needs an expensive machine.

So what do you think about the use of both of them? When to pick Redis? When to pick MongoDB?


Content Type	Original Author	Original Content on Stackoverflow
Question	CamelCamelCamel	View Question on Stackoverflow
Solution 1 - Mongodb	svjson	View Answer on Stackoverflow

MongoDB :: are Mongo IDs unique across collections?

Mongodb Problem Overview

Mongodb Solutions

Solution 1 - Mongodb

What events are triggered when calling fetch() on a Backbone.js collection?

C++ cast to derived class

Attributions