php-doc-en/reference/mongo/queries.xml

<?xml version="1.0" encoding="utf-8"?>
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<section xml:id="mongo.queries" xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink">
 <title>Querying MongoDB</title>

 <section>
  <title>Querying by _id</title>
  <para>
   Every object inserted is automatically assigned a unique _id field, which is
   often a useful field to use in queries.
  </para>
  <para>
   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:

   <programlisting role="php">
<![CDATA[
<?php

$person = array("name" => "joe");

$people->insert($person);

// now $joe has an _id field
$joe = $people->findOne(array("_id" => $person['_id']));

?>
]]>
   </programlisting>
  </para>
  <para>
   Unless the user has specified otherwise, the _id field is a 
   <classname>MongoId</classname>.  The most common mistake is attepting to use 
   a string to match a <classname>MongoId</classname>.  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:

   <programlisting role="php">
<![CDATA[
<?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));

?>
]]>
   </programlisting>
  </para>
 </section>

 <section>
  <title>Arrays</title>

  <para>
   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:

   <programlisting>
<![CDATA[
{ "_id" : ObjectId("4b06c282edb87a281e09dad9"), "awards" : ["gold", "silver", "bronze"]}
]]>
   </programlisting>

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

   <programlisting role="php">
<![CDATA[
<?php

  $cursor = $collection->find(array("awards" => "gold"));

?>
]]>
   </programlisting>
  </para>

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

   <programlisting>
<![CDATA[
{ 
     "_id" : ObjectId("4b06c282edb87a281e09dad9"), 
     "awards" : 
     [
        {
            "first place" : "gold"
        },
        {
            "second place" : "silver" 
        },
        {
            "third place" :  "bronze"
        }
     ]
}
]]>
   </programlisting>

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

   <programlisting role="php">
<![CDATA[
<?php

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

?>
]]>
   </programlisting>

   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).
  </para>
 </section>

</section>

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