TweetFollow Us on Twitter

Introduction to Core Data, Part IV

Volume Number: 22 (2006)
Issue Number: 2
Column Tag: Programming

Introduction to Core Data, Part IV

Storing Fetch Requests in your Data Model

by Jeff LaMarche

In the last Core Data article we talked about creating Fetch Requests programmatically using format strings and predicates. Using format strings to create Fetch Requests is a little bit inelegant, although it does give you a tremendous amount of flexibility. For your day-to-day, run-of-the-mill data fetching needs there is an easier way that gets those format strings out of your compiled code. You can create Fetch Requests right in your data model. This month, we're going to take a quick look at doing just that: We're going to build Fetch Requests and predicates right in Xcode and then look at how we get those Fetch Requests from our code.

Introduction

In Xcode, I went ahead and built a simple data model with a single entity called Person that contains two string attributes called firstName and lastName, and a date attribute called birthDate. I also gave the entity a to-one relationship called spouse that can connect this entity to one other Person entity. I'll be using this simple data model to illustrate my points in the rest of this article. We've already discussed creating data models back in the first part of this series (Intro to Core Data, July, 2005), so I won't be guiding you through creating the data model this month. If you create a new project in Xcode using the Core Data Application project template, you should be able to quickly create the same data model yourself, or you can simply download my project from the MacTech FTP site. You might want to do that before you continue with the article, as I'll be making frequent references to this data model, however it is a simple enough data model that you can probably follow along without it.

Stored Fetch Requests

If you search Apple's documentation for information on how to store Fetch Requests in your data model, you might come to the conclusion that it simply can't be done. The ability to store Fetch Requests as part of the data model, although present in Core Data's predecessor Enterprise Objects Framework, seemed to be absent from the data model editor in Xcode when Core Data was first released. It was actually there, but it just wasn't very obvious; you had to change the view settings for the properties pane in order to see or work with Fetch Requests (Figure 1). The official documentation spends so little time on the topic that it's very easy to miss it.


Figure 1. Getting to the fetch requests stored in your data model.

With the recent release of Xcode 2.1, however, the eagle-eyed among you have probably noticed that a fourth option has been added to the property pane's add (plus sign) pop-up menu in the data model editor (Figure 2). Unlike with earlier releases, this option now shows up regardless of whether you've changed the pane to show Fetch Requests or not. That makes the functionality a little more obvious, and a little easier to access.


Figure 2. You can now add a fetch request when looking at the properties pane in any view mode.

If you decide to store fetch requests in your data model (and you should), you still have to know about predicates and format strings. All that information I gave you in the Core Data III article back in November was not for naught. Without a solid understanding of format strings and predicates, you wouldn't be able to use the functionality we're going to go over this month. See? There is a method to my madness.

The Predicate Builder

The way that we build Fetch Requests in Xcode is to use the Predicate Builder. This is a tool that is built into Xcode's data model editor, and is used for visually creating predicates (and the format string that they're based on). Why don't you go ahead and select the Person entity in your data model, then press the little plus sign in the properties pane (the top-middle pane). Select Add Fetch Request from the menu that pops up (Figure 2). You'll get a new Fetch Request, which you should go ahead and rename to olderThanSpouse in the top right pane. We are going to create a predicate that brings back all people who have a spouse and are older than them.

When you edit the name of this Fetch Request, you should see a button labeled Edit Predicate down below where you change the name. This button will call up the Predicate Builder. The large text field above the Edit Predicate button will show you the format-string associated with any predicate you build with the Predicate Builder. Although it looks like you should be able to edit the format string there, you actually can't, though you can select it and copy to the clipboard.


Figure 3. You can get to the predicate builder by pressing the Edit Predicate button when the fetch request is selected in the properties pane.

Go ahead and click that button to bring up the Predicate Builder. This interface probably looks familiar to you, at least if you've ever set up a rule in Mail, or defined a smart playlist in iTunes or a smart album in iPhoto. Predicates are built using a fairly standard-issue Apple rules interface (Figure 4). Almost. It's a very functional interface, but it's not as intuitive as the ones you've probably used. There's actually a little bit of hidden functionality in there, so let's take a closer look at it.


Figure 4. The Predicate Builder

Starting from the left side, the first thing you'll see is a pop-up menu. This is the key pop-up menu, sometimes simply referred to as the left hand side menu. This is where you define what you're comparing or, in other words, the left side of the format string. Most often, you're going to select one of the attributes of your entity. If you click the pop-up menu, you'll see that you have more options to select from than just the attributes of the current entity, however (Figure 5).


Figure 5. The key pop-up menu.

The top-most item in the menu is labeled Expression. You won't find this option well-documented anywhere unfortunately, but it's a very handy one nonetheless. This option changes the whole line to a single editable text field where you can plug in a format string. Why would you want to do that? Well, there might be times when you need to access a property using key-value coding that the data model editor doesn't know about, perhaps because you've created an attribute and left its type as undefined. Basically, if you know about an attribute that Xcode doesn't know about for any reason, you can use this option to manually enter the format string for one part of the predicate. In practice, you probably won't have to use this very often, but it's good to know it's there in case you ever need it.

The next option, labeled Select Key... brings up a browser to let you select an attribute. This might seem a little redundant, since the individual attributes of this entity are also available right in the pop-up menu, but you might have noticed that the spouse relationship was not accessible there, because it's not an attribute. Although this option can be used just to select an attribute of this entity, the primary reason it exists is so that you can drill down into other entities through this entity's relationships. Let's say we wanted to create a fetch request based on the birthday of the person's spouse. In that case, you could use the Select Key... option, click on the spouse entry in the left-most column, then click on the birthDate field in the second column (Figure 6). Don't actually do that, however. Click the Cancel button; I don't want you to actually select the spouse's birthDate field here.


Figure 6. Selecting attributes of a relationship using the Select Key... option.

The next few options are pretty self-explanatory; they are simply the attributes of this entity that you can select (firstName, lastName, birthDate) The options in the next section below the attributes allow you to add additional criteria to this rule. Here's where we start to see a deviation from the standard-issue Apple rules dialog. In most rules dialogs, such as iTunes' smart playlist dialog (Figure 7), you define a simple list of criteria and choose to require either all of the criteria, or any single criteria in the list.


Figure 7. The smart playlist rules dialog from iTunes.

The Predicate Builder allows you to create much more complex and robust rules than the rule dialogs in Apple's consumer applications. You can use all three of the standard Boolean operators (and, or, not), and choose a different operator for each pair of operations. You can also set precedence by creating a hierarchy of criteria.

The four Add options under the key pop-up menu all insert another criterion into the predicate, and function almost exactly the same. The only difference in the options, is which of the Boolean operators is used, although that can be changed after the fact. The bottom-most Add option is the same as pressing the plus button on the right-hand side of the row or the Add AND option. You can look at Figure 8 to get an idea of the sort of complex rules you can build using the Predicate Builder. The last option on the left-hand side menu simply removes the row and functions exactly like pressing the minus button on the right-hand side of the row.


Figure 8. Crafting a complex Fetch Request in the Predicate Builder.

As we move to the right, the next pop-up menu is the operator to be used for this row. The available operators change depending on the type of attribute selected in the key pop-up menu. Different options are presented, for example, when you select a string attribute than when you select a number or date attribute.

Last, but not least, the next field over is simply the value to be compared when evaluating this criterion. The example in Figure 8 shows a number of comparisons being made, but they're all being made to constant values typed right into this field. That's useful, but not always what you want. To be really useful, you'd need to be able to plug in values at runtime or, perhaps, compare one attribute to another. But there doesn't seem to be any way to enter anything other than a constant here, does there?

Well, here's a bit of hidden functionality. Go ahead and either right-click or control-click in the space to the left of the minus button that's used to delete the row. Lo and behold, you'll be presented with a contextual menu with some new options(Figure 9).


Figure 9. The hidden contextual menu.

The bottom five options are exactly the same as ones you saw in the left-hand key pop-up menu. They add or remove the current row. The top three options, however, are the magic options. They allow you to select the types values for the right-hand side of the equation. The default value, as you've seen, is Constant, which allows you to specify the comparison value right in the Predicate Builder.

Let's skip the second option (Variable) for just a second. We'll come back to it in a moment, but it's more involved than the other two, so let's look at the third option first. Go ahead and select Key. Doing that, will change the place where you used to be able to enter a constant value into another pop-up menu that looks identical to the left-hand side pop-up menu. This is the right-hand side key pop-up menu. This can be useful in many situations. We stated that we were going to create a Fetch Request to retrieve all people who are older than their spouse. Easy enough. Just choose Select Key... from the right-hand side pop-up menu and drill down to the spouse's birthDate as we did a moment ago on the left-hand side. Next, change the operator pop-up menu to greater-than and make sure birthDate is selected in the left-hand key pop-up menu. (Figure 10).


Figure 10. Comparing a key to another key

Go ahead and click the OK button; this Fetch Request is done. You can take a look in the Predicate: field to see the format string you just created. Now, let's create a little more complex Fetch Request. Go ahead and click the plus button in the lower left of the Fetch Request pane (the top-middle pane of the data model editor). When you added a Fetch Request before, the Property pane turned into the Fetch request pane, so this time, you can just click the plus button and don't have to select from a pop-up menu. When the new Fetch Request comes up, rename it to birthDateBetween, then click the Edit Predicate button to get back to the Predicate Builder. We're going to build a Fetch Request to retrieve all people with a birth date between two values, but we don't want to specify those two values until runtime.

Let's now talk about the second option on the contextual menu: Variable. If you control-click to the left of the minus button again and select that option, you'll get an editable text field labeled Variable (Figure 11). This is where the juju happens, baby. In this field, we can specify a variable name and later we'll be able to replace that variable with another value in our code. This is called a substitution variable or sometimes a bind variable. This is what makes stored Fetch Requests flexible enough to use for just about everything. If you specify a value here - which can be just about any string value you wish to use - you will then be able to substitute any other value of your choosing when you retrieve the Fetch Request from your data model.

If you're playing along at home, go ahead and change the operator pop-up menu from = to within, which will add a second row to this criteria. The new row is not a new criteria, but just a second parameter to the existing one. The within operator allows you to search based on a date attribute that falls between two other date values. Control-click to the right of the new row that got entered and change it also to variable. Now, go ahead and assign each of those variables a name. I chose FROMDATE and TODATE, as you can see from Figure 11. Once you've got it set up, click the OK button.


Figure 11. Specifying a substitution variable in Predicate Builder.

After the Predicate Builder is dismissed, go ahead and look at that large text field labeled Predicate:. That will show you the format string that will be used for creating this Fetch Request. Notice the dollar sign in front of FROMDATE and THRUDATE? That's how substitution variables are specified in format strings.

Go ahead and save your data model and let's take a look at how we can pull our Fetch Request out of the data model instead of having to create them from scratch, as we did in the previous article.

Getting Fetch Requests Out Of the Data Model

Technically speaking, you aren't storing a Fetch Request in your data model, even though it's usually referred to that way. Your data model is not like a nib file, where serialized objects are being stored. Instead, what's really happening is you are creating a template from which a new Fetch Request can be instantiate at run time. This is a relatively straight forward process.

One thing that might trip you up is the fact that you don't use NSMangedObjectContext to retrieve your Fetch Requests. If you've been using Core Data for a little while, you've probably gotten accustomed to using the context for retrieving, inserting, and editing your data. A Fetch Request, however, is not data. It's part of your data model. Therefore, instead of using the context, you have to use your managed object model (NSManagedObjectModel) to retrieve your Fetch Requests. Fortunately, the managed object model is available through an accessor method that gets created automatically for you when you use one of the Core Data project templates. In a Core Data Application project, you can get the managed object model using the managedObjectModel method of the application delegate class (applicationName_AppDelegate). In a Core Data document-based application, you use the same method, but call it on your document class instead (which inherits the method from NSPersistentDocument).

Once you have a reference to your managed object model, it's simply a matter of asking it for your Fetch Request by name. If you aren't using substitution variables, it's very easy. Here's how we would get that first Fetch Request we created:

Getting a Fetch Request from your data model

NSManagedObjectModel *model = [self managedObjectModel];
NSFetchRequest *fetch = 
   [model fetchRequestTemplateForName:@"olderThanSpouse"];

That's all there is to that one. After this call, you're ready to fetch data from the context.

On the other hand, if you have specified substitution variables in your Fetch Request, then you have to build a dictionary containing the substitution variable names as the key values, and the values you wish to replace them with as the corresponding objects. In our case, that means creating an NSDictionary with two entries. One with a key value of @"FROMDATE" and another with a key value of @"TODATE". The objects to be passed for both of these keys need to be NSDate object instances since we are comparing with a date field. If we were comparing a numeric field, we'd use NSNumber, and if were comparing a string field, we would use an NSString. Here's an example of getting our second Fetch Request out of the data model:

Getting a Fetch Request with substitution variables

NSManagedObjectModel *model = [self managedObjectModel];
NSDate *fromDate = [NSCalendarDate 
   dateWithNaturalLanguageString:@"1/1/2004"];
NSDate *toDate = [NSCalendarDate 
   dateWithNaturalLanguageString:@"12/31/2004"];
NSDictionary *subs = [NSDictionary 
   dictionaryWithObjectsAndKeys:fromDate, @"FROMDATE", 
   toDate, @"TODATE", nil];
NSFetchRequest *fetch = 
   [model fetchRequestFromTemplateWithName:
   @"birthDateBetween" substitutionVariables:subs];

Once either of these two chunks of code fires, you will have a pointer to a Fetch Request. It is exactly the same as if you had created it using a format string and a predicate like we did in the previous article, but with less code and without having the format string inside your compiled application.

Conclusion

Storing the definition of your Fetch Requests inside your data model has many advantages. You can use the Predicate Builder to more easily build complex criteria it allows you to move potentially complex format strings out of your compiled code and into your data model. It also allows you to use bind variables to let you alter your Fetch Requests at runtime based on user-provided data.

These tools provide you with the ability to create and use just about any type of Fetch Request you might need right in your data model, and there is very little reason in most situation not to use it. Unfortunately, the fact that the functionality was sort of squirreled away where it wasn't obvious means it is probably being underused, but now that you where it is and how to use it, you can go ahead and get all those yucky format strings and predicates out of your code.


Jeff LaMarche wrote his first line of code in Applesoft Basic on a Bell & Howell "Darth Vader" Apple ][+ in 1980 and he's owned at least one Apple computer at all times since. In addition to writing, Jeff codes in a variety of languages, with Cocoa Objective-C being, by far, his favorite. Feel free to drop him a line anytime at jeff_lamarche@mac.com.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

BBEdit 11.1.1 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
CrossOver 14.1.3 - Run Windows apps on y...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Little Snitch 3.5.3 - Alerts you about o...
Little Snitch gives you control over your private outgoing data. Track background activity As soon as your computer connects to the Internet, applications often have permission to send any... Read more
OmniGraffle Pro 6.2.3 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
OmniFocus 2.2 - GTD task manager with iO...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more
Cocktail 8.4 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
PDFKey Pro 4.3 - Edit and print password...
PDFKey Pro can unlock PDF documents protected for printing and copying when you've forgotten your password. It can now also protect your PDF files with a password to prevent unauthorized access and/... Read more
Kodi 15.0.beta1 - Powerful media center...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
DiskCatalogMaker 6.4.12 - Catalog your d...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast. Finder-like intuitive look and feel. Super-fast search algorithm. Can compress catalog data... Read more
Macs Fan Control 1.3.0.0 - Monitor and c...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more

Moleskine Timepage – Calendar for iCloud...
Moleskine Timepage – Calendar for iCloud, Google & Exchange 1.0 Device: iOS iPhone Category: Productivity Price: $4.99, Version: 1.0 (iTunes) Description: The most elegant calendar for your pocket and wrist, Timepage is a... | Read more »
QuizUp Gets Social in its New Update
Plain Vanilla Corp has released a new and improved version of their popular trivia game, QuizUp. The app now emphasizes social play so you can challenge friends from all over the world. [Read more] | Read more »
The Deep (Games)
The Deep 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Swipe Controls Delve into the deep in this retro rogue-like! Swipe to move your diver around and keep away from the enemies as you... | Read more »
Battle of Gods: Ascension (Games)
Battle of Gods: Ascension 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: TURN-BASED TACTICAL COMBATFight tactical battles against the forces of Hades! In Battle of Gods: Ascension you play... | Read more »
Shadowmatic's Latest Update Adds a...
Shadowmatic's shadowy shadow-ness is getting a little shadowy-er thanks to a recent update that adds an Arcade Mode. [Read more] | Read more »
Sunrise Calendar and Slack Have Assimila...
Wunderlist is perhaps one of the most populat and beloved productivity apps on the App Store - and now it's gone and incorporated itself into other useful services like Sunrise Calendar and Slack. [Read more] | Read more »
Crossy Road Devs Hipster Whale are Bring...
Hipster Whale, the minds behind the rather popular (and rather great) Crossy Road, have teamed-up with Bandai Namco to create PAC-MAN 256: an absolutely bonkers looking maze runner chaser thing. | Read more »
Meet the New Spotify Music
Spotify Music  has a lot going on. They're introducing 3 new modes to serve all your musical needs, with the "Now" start page  gives you curated playlists based on your particular tastes. As you listen the app will learn more about your tastes and... | Read more »
What the Apple Watch Gets Right, and Wha...
| Read more »
Celebrate PAC-MAN's 35th Birthday W...
BANDAI NAMCO Entertainment America is celebrating PAC-MAN's 35th anniversary by releasing updates for PAC-MAN and PAC-MAN Lite for iOS. [Read more] | Read more »

Price Scanner via MacPrices.net

Apple refurbished 2014 13-inch Retina MacBook...
The Apple Store has Apple Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and... Read more
What Would the ideal Apple Productivity Platf...
For the past four years I’ve kept a foot in both the Mac and iPad camps respectively. my daily computing hours divided about 50/50 between the two devices with remarkable consistency. However, there’... Read more
PageMeUp 1.2.1 Ten Dollar Page Layout Applica...
Paris, France-based Softobe, an OS X software development company, has announced that their PageMeUp v. 1.2.1, is available on the Mac App Store for $9.99. The license can be installed on up to 5... Read more
Eight New Products For USB Type-C Application...
Fresco Logic, specialists in advanced connectivity technologies and ICs, has introduced two new product families targeting the Type-C connector recently introduced across a number of consumer... Read more
Scripps National Spelling Bee Launches Buzzwo...
Scripps National Spelling Bee fans can monitor the action at the 2015 Spelling Bee with the new Buzzworthy app for iOS, Android and Windows mobile devices. The free Buzzworthy app provides friendly... Read more
13-inch 2.5GHz MacBook Pro on sale for $120 o...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $979 including free shipping plus NY sales tax only. Their price is $120 off MSRP, and it’s the lowest price for this model (except for Apple’... Read more
27-inch 3.3GHz 5K iMac on sale for $1899, $10...
B&H Photo has the new 27″ 3.3GHz 5K iMac on sale for $1899.99 including free shipping plus NY tax only. Their price is $100 off MSRP. Read more
Save up to $50 on iPad Air 2, NY tax only, fr...
B&H Photo has iPad Air 2s on sale for up to $50 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469 $30 off - 64GB iPad Air 2 WiFi: $549.99 $50 off - 128GB iPad... Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
New 13-inch 2.9GHz Retina MacBook Pro on sale...
B&H Photo has the 13″ 2.9GHz/512GB Retina MacBook Pro on sale for $1699.99 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest price for this model from... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Watch SW Application Project Manager...
**Job Summary** The Apple Watch software team is looking for an Application Engineering Project Manager to work on new projects for Apple . The successful candidate Read more
Engineering Manager for *Apple* Maps on the...
…the Maps App Team get to take part in just about any new feature in Apple Maps, often contributing a majority of the feature work. In our day-to-day engineering work, we Read more
Senior Software Engineer - *Apple* SIM - Ap...
Changing the world is all in a day039s work at Apple . If you love innovation, here039s your chance to make a career of it. You039ll work hard. But the job comes with Read more
Lead *Apple* Solutions Consultant - Retail...
**Job Summary** Job Summary The Lead ASC is an Apple employee who serves as the Apple business manager and influencer in a hyper-business critical Reseller's store Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.