Requests in CoreData with aggregate functions and grouping in one line
Almost everyone who uses CoreData sooner or later has to create queries with aggregate functions and groupings . However, the syntax of such queries in CoreData is difficult to understand and unnecessarily verbose.
Using the query designer, we can, for example, make such a query:
This query is equivalent to:
In this article I would like to talk about a small library for working with CoreData, which appeared as a summary of my modest development experience under iOS. The library is available in cocoapods .
The core of the library is the singlelton class ALCoreDataManager, which is responsible for initializing and connecting the CoreData stack and returns NSManagedObjectContext. I note that this is a completely ordinary thing and there are a lot of analogues in this regard. All the goodies are contained in the ALFetchRequest + QueryBuilder category and in the class ALManagedObjectFactory. But first things first.
')
We will assume that the Product model is defined as:
Using the query designer , we can form, for example, the following queries:
The execute method is used directly to execute the query. To get the generated NSFetchRequest, use the request method. For example,
Upon request, you can get an object that implements the UITableViewDataSource protocol and is controlled by NSFetchedResultsController :
This will save us from writing boring code to implement the UITableViewDataSource protocol and the delegate for NSFetchedResultsController. A similar object can be obtained for a UICollectionViewDataSource using the collectionViewDataSource method.
Creating and deleting objects is possible using this API :
The last thing to note is that you must overload the + entityName method if the Entity Name of some ManagedObject does not match its Class Name (of course, this must be done in the appropriate category).
Let us demonstrate the profit from using the library with an example . After downloading and unzipping the library, you must install the dependencies:
In Storyboard, everything is quite simple:
The first TableViewController displays a list of all Products; the second displays information on the selected Product, which can be edited there as well.
The table is populated using the ALTableViewDataSource mentioned earlier:
Believe it or not, this is all the code for the tableview.
By clicking on Add , we create an element, saying:
We will show some statistical information by clicking on Statistics :
After selecting the type of statistics, the code works:
This is the whole code with aggregator functions (for comparison, request with aggregate function (stackoverflow) ).
After execution, we get the following AlertView:
Request generation begins with a call:
That is, it simply leads to the creation of NSFetchRequest:
Virtually all of the builder code is in ALFetchRequest + QueryBuilder.m .
Each of the calls of the form
It simply adds the necessary settings to the created NSFetchRequest, for example:
Methods execute and request :
We can assume that this is just syntactic sugar for NSFetchRequest. Obviously, overhead is almost zero. Slightly more examples can be found in the tests .
At this story I hasten to finish. Thank you for attention.
Using the query designer, we can, for example, make such a query:
NSDictionary *productTotalSumAndAveragePriceGroupedByCountries = [[[[[Product all ] aggregatedBy:@[ @[kAggregateSum, @"amount"], @[kAggregatorAverage, @"price"]] ] groupedBy:@[@"country"] ] having:predicate ] execute]; This query is equivalent to:
Request in coredata
NSFetchRequest *fetchRequest = [[ALFetchRequest alloc] init]; fetchRequest.managedObjectContext = managedObjectContext; NSString *entityName = @"Product"; NSEntityDescription *entity = [NSEntityDescription entityForName:entityName inManagedObjectContext:managedObjectContext]; [fetchRequest setEntity:entity]; [fetchRequest setIncludesPendingChanges:YES]; // sum amount NSExpression *fieldExp1 = [NSExpression expressionForKeyPath:@"amount"]; NSExpression *agrExp1 = [NSExpression expressionForFunction:agr arguments:@[fieldExp1]]; NSExpressionDescription *resultDescription1 = [[NSExpressionDescription alloc] init]; NSString *resultName1 = @"sumAmount"; [resultDescription1 setName:resultName1]; [resultDescription1 setExpression:agrExp1]; [resultDescription1 setExpressionResultType:NSInteger64AttributeType]; // average price NSExpression *fieldExp2 = [NSExpression expressionForKeyPath:@"price"]; NSExpression *agrExp2 = [NSExpression expressionForFunction:agr arguments:@[fieldExp1]]; NSExpressionDescription *resultDescription2 = [[NSExpressionDescription alloc] init]; NSString *resultName2 = @"sumAmount"; [resultDescription2 setName:resultName2]; [resultDescription2 setExpression:agrExp2]; [resultDescription2 setExpressionResultType:NSInteger64AttributeType]; // country NSDictionary *availableKeys = [entity attributesByName]; NSAttributeDescription *country = [availableKeys valueForKey:@"country"]; fetch.propertiesToFetch = [NSArray arrayWithObjects:country, resultDescription1, resultDescription2, nil]; fetch.propertiesToGroupBy = [NSArray arrayWithObject:country]; fetch.resultType = NSDictionaryResultType; NSError *error; NSManagedObjectContext *managedObjectContext = self.managedObjectContext; NSArray *fetchedObjects = [managedObjectContext executeFetchRequest:self error:&error]; if (!fetchedObjects || error) { NSLog(@"Error: Execution of the fetchRequest: %@, Failed with Description: %@",self,error); } return fetchedObjects; In this article I would like to talk about a small library for working with CoreData, which appeared as a summary of my modest development experience under iOS. The library is available in cocoapods .
The core of the library is the singlelton class ALCoreDataManager, which is responsible for initializing and connecting the CoreData stack and returns NSManagedObjectContext. I note that this is a completely ordinary thing and there are a lot of analogues in this regard. All the goodies are contained in the ALFetchRequest + QueryBuilder category and in the class ALManagedObjectFactory. But first things first.
')
Library features
We will assume that the Product model is defined as:
@interface Product : NSManagedObject @property (nonatomic, retain) NSString *title; @property (nonatomic, retain) NSNumber *price; @property (nonatomic, retain) NSNumber *amount; @property (nonatomic, retain) NSString *country; @end Requests
Using the query designer , we can form, for example, the following queries:
NSArray *allProducts = [[Product all] execute]; NSArray *productsFilteredWithPredicate = [[[Product all] where:predicate] execute]; NSArray *singleProduct = [[[[Product all] where:predicate] limit:1] execute]; NSArray *onlyDistinctProductTitles = [[[[Product all] properties:@[@"title"]] distinct] execute]; NSArray *countProducts = [[[[Product all] where:predicate] count] execute]; // NSInteger count = [[countProducts firstObject] integerValue]; NSArray *productsOrderedByTitleAndPrice = [[[Product all ] orderedBy:@[ @[@"title", kOrderDESC], @[@"price", kOrderASC], @[@"amount"]] ] execute]; NSArray *totalAmountAndAveragePriceForProducts = [[[[[Product all ] aggregatedBy:@[ @[kAggregateSum, @"amount"], @[kAggregateAverage, @"price"]] ] groupedBy:@[@"country"] ] having:predicate ] execute]; The execute method is used directly to execute the query. To get the generated NSFetchRequest, use the request method. For example,
NSFetchRequest *request = [[[Product all] orderedBy:@[@"title", @"price"]] request]; NSManagedObjectContext *context = [ALCoreDataManager defaultManager].managedObjectContext; NSFetchedResultsController *controller = [[NSFetchedResultsController alloc] initWithFetchRequest:request managedObjectContext:context sectionNameKeyPath:nil cacheName:nil]; [controller performFetch:nil]; UITableViewDataSource
Upon request, you can get an object that implements the UITableViewDataSource protocol and is controlled by NSFetchedResultsController :
ALTableViewDataSource *dataSource = [[[Product all] orderedBy:@[kTitle, kPrice]] tableViewDataSource]; self.dataSource.tableView = self.tableView; This will save us from writing boring code to implement the UITableViewDataSource protocol and the delegate for NSFetchedResultsController. A similar object can be obtained for a UICollectionViewDataSource using the collectionViewDataSource method.
Creating and deleting objects
Creating and deleting objects is possible using this API :
Product *a = [Product create]; Product *b = [Product createWithDictionary:@{ @"title" : @"best product" }]; // Factory- NSManagedObjectContext *context = [ALCoreDataManager defaultManager].managedObjectContext; ALManagedObjectFactory *factory = [[ALManagedObjectFactory alloc] initWithManagedObjectContext:context]; Product *c = [Product createWithDictionary:nil usingFactory:factory]; c.title = @"best product 2"; Product *d = [Product createWithDictionary:@{ @"title" : @"best product 3", @"price" : @(100) } usingFactory:factory]; [d remove]; // The last thing to note is that you must overload the + entityName method if the Entity Name of some ManagedObject does not match its Class Name (of course, this must be done in the appropriate category).
@implementation Product + (NSString*)entityName { return @"AnItem"; } @end Example
Let us demonstrate the profit from using the library with an example . After downloading and unzipping the library, you must install the dependencies:
cd / Users / you / Downloads / ALCoreDataManager-master / Example pod install
In Storyboard, everything is quite simple:
The first TableViewController displays a list of all Products; the second displays information on the selected Product, which can be edited there as well.
The table is populated using the ALTableViewDataSource mentioned earlier:
- (void)viewDidLoad { [super viewDidLoad]; self.dataSource = [[[Product all] orderedBy:@[kTitle, kPrice]] tableViewDataSource]; __weak typeof(self) weakSelf = self; self.dataSource.cellConfigurationBlock = ^(UITableViewCell *cell, NSIndexPath *indexPath){ [weakSelf configureCell:cell atIndexPath:indexPath]; }; self.dataSource.reuseIdentifierBlock = ^(NSIndexPath *indexPath){ return TableViewCellReuseIdentifier; }; self.dataSource.tableView = self.tableView; } Believe it or not, this is all the code for the tableview.
By clicking on Add , we create an element, saying:
[Product createWithFields:@{ kTitle : title, kPrice : @(0), kAmount : @(0) } usingFactory:[ALManagedObjectFactory defaultFactory]]; We will show some statistical information by clicking on Statistics :
After selecting the type of statistics, the code works:
ALFetchRequest *request = nil; switch (st) { case ALStatsTypeTotalAmount: request = [[Product all] aggregatedBy:@[@[kAggregatorSum, kAmount]]]; break; case ALStatsTypeMedianPrice: request = [[Product all] aggregatedBy:@[@[kAggregatorAverage, kPrice]]]; break; default: break; } NSArray *result = [request execute]; // request NSDictionaryResultType NSDictionary *d = [result firstObject]; // : // { // sumAmount = 1473; // } This is the whole code with aggregator functions (for comparison, request with aggregate function (stackoverflow) ).
After execution, we get the following AlertView:
How it works
Request generation begins with a call:
+ (ALFetchRequest*)allInManagedObjectContext:(NSManagedObjectContext*)managedObjectContext; + (ALFetchRequest*)all; // allInManagedObjectContext defaultContext That is, it simply leads to the creation of NSFetchRequest:
+ (ALFetchRequest*)allInManagedObjectContext:(NSManagedObjectContext*)managedObjectContext { ALFetchRequest *fetchRequest = [[ALFetchRequest alloc] init]; fetchRequest.managedObjectContext = managedObjectContext; NSEntityDescription *entity = [self entityDescriptionWithMangedObjectContext:managedObjectContext]; [fetchRequest setEntity:entity]; [fetchRequest setIncludesPendingChanges:YES]; return fetchRequest; } Virtually all of the builder code is in ALFetchRequest + QueryBuilder.m .
Each of the calls of the form
[[[Product all] orderedBy:@[kTitle, kPrice]] limit:1]; It simply adds the necessary settings to the created NSFetchRequest, for example:
- (ALFetchRequest*)limit:(NSInteger)limit { self.fetchLimit = limit; return self; } Methods execute and request :
- (NSArray*)execute { NSError *error; NSManagedObjectContext *managedObjectContext = self.managedObjectContext; NSArray *fetchedObjects = [managedObjectContext executeFetchRequest:self error:&error]; if (!fetchedObjects || error) { NSLog(@"Error: Execution of the fetchRequest: %@, Failed with Description: %@",self,error); } return fetchedObjects; } - (NSFetchRequest *)request { return (NSFetchRequest*)self; } We can assume that this is just syntactic sugar for NSFetchRequest. Obviously, overhead is almost zero. Slightly more examples can be found in the tests .
At this story I hasten to finish. Thank you for attention.
Source: https://habr.com/ru/post/265319/
All Articles