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## Querying

## Querying by _id

Every object inserted is automatically assigned a unique _id field, which is often a useful field to use in queries.

Suppose that we wish to find the document we just inserted. Inserting adds and _id field to the document, so we can query by that:

`<?php
$person = array("name" => "joe");
$people->insert($person);
// now $joe has an _id field
$joe = $people->findOne(array("_id" => $person['_id']));
?>`

Unless the user has specified otherwise, the _id field is a MongoId. The most common mistake is attepting to use a string to match a MongoId. Keep in mind that these are two different datatypes, and will not match each other in the same way that the string "array()" is not the same as an empty array. For example:

`<?php
$person = array("name" => "joe");
$people->insert($person);
// convert the _id to a string
$pid = $person['_id'] . "";
// FAILS - $pid is a string, not a MongoId
$joe = $people->findOne(array("_id" => $pid));
?>`

## Arrays

Arrays are special in a couple ways. First, there are two types that MongoDB uses: "normal" arrays and associative arrays. Associative arrays can have any mix of key types and values. "Normal" arrays are defined as arrays with ascending numeric indexes starting at 0 and increasing by one for each element. These are, typically, just your usual PHP array.

For instance, if you want to save a list of awards in a document, you could say:

`<?php
$collection->save(array("awards" => array("gold", "silver", "bronze")));
?>`

Queries can reach into arrays to search for elements. Suppose that we wish to find all documents with an array element of a given value. For example, documents with a "gold" award, such as:

    { "_id" : ObjectId("4b06c282edb87a281e09dad9"), "awards" : ["gold", "silver", "bronze"]}

This can be done with a simple query, ignoring the fact that "awards" is an array:

`<?php
$cursor = $collection->find(array("awards" => "gold"));
?>`

Suppose we are querying for a more complex object, if each element of the array were an object itself, such as:

    {
         "_id" : ObjectId("4b06c282edb87a281e09dad9"),
         "awards" :
         [
            {
                "first place" : "gold"
            },
            {
                "second place" : "silver"
            },
            {
                "third place" :  "bronze"
            }
         ]
    }

Still ignoring that this is an array, we can use dot notation to query the subobject:

`<?php
$cursor = $collection->find(array("awards.first place" => "gold"));
?>`

Notice that it doesn't matter that there is a space in the the field name (although it may be best not to use spaces, just to make things more readable).

You can also use an array to query for a number of possible values. For instance, if we were looking for documents "gold" or "copper", we could do:

`<?php
$cursor = $collection->find(array("awards" => array('$in' => array("gold", "copper"))));
?>`

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