tuesdayDeveloper;

Haha, I just thought how weird that title would look to anyone who isn’t a programmer (why would you want to ask sting something).

Stupid jokes aside, I thought it would be important to write down how to query Objects, like dates or people in Google App Engine. The concept is very simple, though you may have to put in a little extra thought. For example, lets query a date. We want to design a calendar that will store events for different days, and we want these events to have specific times (that should be obvious). How do we search for these when we need to?

Say we’re searching for all events on March 22nd. This will include my “Wake up” alert at 6:30 am as well as my “Appointment with Mr. Money” from 10:00 am to 2:00 pm. In any query language you’ll have to say, “Hey, db, I want everything between 12:00 am and 11:59:59 on March 22nd.” To do this in Google App Engine you’ll do the following, (you’ll also do similar things with other Objects). Note, you may want to read my post on Google App Engine Datastore to get some of the basics.

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//...
//open your persistence manager
PersistenceManager pm = PMF.get().getPersistenceManager()
//create a new Query
Query eventQuery = pm.newQuery(Event.class);
//set the filter for the Query. Notice that there is a range the date must fall in, this may or may not be necessary on other Objects.
eventQuery.setFilter("startDate <= startDateParam && startDate <= endDateParam");
//declare the parameters of the Query
eventQuery.declareParameters("java.util.Date startDateParam, java.util.Date endDateParam"); //notice the full address of the Date class
//set any ordering you want
eventQuery.setOrdering("startDate");
//execute the query
List<Event> events = (List<Event>) eventQuery.execute(startDate, endDate);
//...

You’ll notice that I used java.util.Date even though a great majority of the methods in this class are deprecated. The reason why is because of a bug I came across (I feel okay calling it a bug since other people have had problems with it and since when I changed the class name it made no difference but when I changed my GregorianCalendars to Dates it worked fine.) Maybe they will fix it and you won’t have the same problem.

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I must admit that I was extremely confused when I was positioning divs with css today. I had two divs inside of another div. Something like the following.

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<div id="container">
    <div id="blue">This is the blue div.</div>
    <div id="red">This is the red div.</div>
</div>

The css for the page was as follows:

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div#container {
    width:80%;
    margin-left:auto;
    margin-right:auto;
    border:solid #000000;
    border-width:5px;
}
 
div#blue {
    background-color:blue;
    width:50%;
    float:left;
}
 
div#red {
    background-color:red;
    width:50%;
    float:left;
}

What I expected and what I got were two different things.

Expected:

Actual:

Why did this happen? Well, apparently if you use the float option in css, it doesn’t mean that the divs will still look like they are in the container. The easiest way to fix this is with a   or a <br /> somewhere after the second div, though that also has its consequences. Probably the best way i’ve found is to use a div with a clear attribute:

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<div id="container">
    <div id="blue">This is the blue div.</div>
    <div id="red">This is the red div.</div>
</div>

With the css for the div:

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#clear {
    clear:both;
}

That works quite nicely!

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Before you deploy your app in Google App Engine, you must configure your app to include static files. Important files—like your css files! So, if you want to have your application look the same in testing and in deployment, then follow these steps.

Open appengine-web.xml
The first thing you’ll need to do is fine the appengine-web.xml file located in [your program folder] > war > WEB-INF. It should look something like this (in Source view):

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<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
     <application>application</application>
     <version>1</version>
 
     <!-- Configure java.util.logging -->
     <system-properties>
          <property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>
     </system-properties>
     <ssl-enabled>true</ssl-enabled>
     <sessions-enabled>true</sessions-enabled>
</appengine-web-app>

Once it’s open, you’ll need to insert a static-files tag into the file (it doesn’t matter where, but I usually put it after the version attribute). Between the open and close static-files tags put the include tag with apath attribute. It should look like this:

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<static-files>
      <include path="/favicon.ico" />
      <include path="/css/screen.css" />
      <include path="/img/*.png" />
      <include path="/js/*" />
</static-files>

As you can see, you can use the * wildcard so in order to save a lot of potential typing.

All together it should look like this:

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<?xml version="1.0" encoding="utf-8"?>
<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
     <application>prs-app</application>
     <version>1</version>
     <static-files>
	<include path="/favicon.ico" />
  	<include path="/css/screen.css" />
  	<include path="/img/*.png" />
  	<include path="/js/*" />
     </static-files>
 
     <!-- Configure java.util.logging -->
     <system-properties>
         <property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>
     </system-properties>
     <ssl-enabled>true</ssl-enabled>
     <sessions-enabled>true</sessions-enabled>
</appengine-web-app>

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I learned something today while I was debugging some code: It turns out that a child class can only belong to one parent class in the datastore. If you attempt to use a child class in two different places, it may seem to work at first, but then you’ll get an excetpion and error message like this:
java.lang.ClassCastException: oid is not instanceof javax.jdo.identity.ObjectIdentity ...
It was strange, one minute this code was working fine, after I worked on some other code I went back and it stopped working. Here is what I was trying to do:

I’m working on a project that will track real estate listings (Listing) and clients (Person). Each of those Objects need an Address, so I thought I’d just use the same bean like I would in with a relational database table. Turns out, however, you can’t do that in Google’s Datastore.

To quote Max Ross of Google, “Due to the nature of primary keys in the app engine datastore, a [sub]class can only have a single owner. We detect other flavors of this scenario when the entity meta-data is loaded but not this particular variant” (1). Max did say later in the post that he would look into how they could repair that, but I don’t really know if that’s going to happen any time soon.

In order to not have this bug, I’ll have to use two different Addresses for those beans now. Sigh.

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Want to view what information is stored in your datastore in Google App Engine? Go to http://localhost:8888/_ah/admin (or http://localhost:8080/_ah/admin if you are using that port).

Here is what you’ll see:

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