firebase database - work with lists

Work with Lists of Data on iOS

Get a FIRDatabaseReference

To read or write data from the database, you need an instance of 

FIRDatabaseReference

:

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OBJECTIVE-C

@property (strong, nonatomic) FIRDatabaseReference *ref;

self.ref = [[FIRDatabase database] reference];Note: By default, read and write access to your database is restricted so only authenticated users can read or write data. To get started without setting up Authentication, you can configure your rules for public access. This does make your database open to anyone, even people not using your app, so be sure to restrict your database again when you set up authentication.

Reading and writing lists

Append to a list of data

Use the 

childByAutoId

 method to append data to a list in multiuser applications. The 

childByAutoId

 method generates a unique key every time a new child is added to the specified Firebase reference. By using these auto-generated keys for each new element in the list, several clients can add children to the same location at the same time without write conflicts. The unique key generated by 

childByAutoId

 is based on a timestamp, so list items are automatically ordered chronologically.You can use the reference to the new data returned by the 

childByAutoId

 method to get the value of the child's auto-generated key or set data for the child. Calling 

getKey

 on a 

childByAutoId

 reference returns the auto-generated key.You can use these auto-generated keys to simplify flattening your data structure. For more information, see the data fan-out example.

Listen for child events

Child events are triggered in response to specific operations that happen to the children of a node from an operation such as a new child added through the 

childByAutoId

 method or a child being updated through the 

updateChildValues

 method.

Event type	Typical usage
FIRDataEventTypeChildAdded	Retrieve lists of items or listen for additions to a list of items. This event is triggered once for each existing child and then again every time a new child is added to the specified path. The listener is passed a snapshot containing the new child's data.
FIRDataEventTypeChildChanged	Listen for changes to the items in a list. This event is triggered any time a child node is modified. This includes any modifications to descendants of the child node. The snapshot passed to the event listener contains the updated data for the child.
FIRDataEventTypeChildRemoved	Listen for items being removed from a list. This event is triggered when an immediate child is removed.The snapshot passed to the callback block contains the data for the removed child.
FIRDataEventTypeChildMoved	Listen for changes to the order of items in an ordered list. This event is triggered whenever an update causes reordering of the child. It is used with data that is ordered by  queryOrderedByChild  or  queryOrderedByValue .

Each of these together can be useful for listening to changes to a specific node in a database. For example, a social blogging app might use these methods together to monitor activity in the comments of a post, as shown below:

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// Listen for new comments in the Firebase database
[_commentsRef
              observeEventType:FIRDataEventTypeChildAdded
              withBlock:^(FIRDataSnapshot *snapshot) {
                [self.comments addObject:snapshot];
                [self.tableView insertRowsAtIndexPaths:@[
                  [NSIndexPath indexPathForRow:self.comments.count - 1 inSection:kSectionComments]
                ]
                                      withRowAnimation:UITableViewRowAnimationAutomatic];
              }];
// Listen for deleted comments in the Firebase database
[_commentsRef
 observeEventType:FIRDataEventTypeChildRemoved
 withBlock:^(FIRDataSnapshot *snapshot) {
   int index = [self indexOfMessage:snapshot];
   [self.comments removeObjectAtIndex:index];
   [self.tableView deleteRowsAtIndexPaths:@[[NSIndexPath indexPathForRow:index inSection:kSectionComments]]
                         withRowAnimation:UITableViewRowAnimationAutomatic];
 }];

Listen for value events

While listening for child events is the recommended way to read lists of data, there are situations listening for value events on a list reference is useful.Attaching a 

FIRDataEventTypeValue

 observer to a list of data will return the entire list of data as a single DataSnapshot, which you can then loop over to access individual children.Even when there is only a single match for the query, the snapshot is still a list; it just contains a single item. To access the item, you need to loop over the result:

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[_commentsRef
              observeEventType:FIRDataEventTypeValue
              withBlock:^(FIRDataSnapshot *snapshot) {
                // Loop over children
                NSEnumerator *children = [snapshot children];
                FIRDataSnapshot *child;
                while (child = [children nextObject]) {
                  // ...
                }
              }];This pattern can be useful when you want to fetch all children of a list in a single operation, rather than listening for additional child added events.

Sorting and filtering data

You can use the Realtime Database 

FIRDatabaseQuery

 class to retrieve data sorted by key, by value, or by the value of a child. You can also filter the sorted result to a specific number of results or a range of keys or values.Note: Filtering and sorting can be expensive, especially when done on the client. If your app uses queries, define the 

.indexOn

 rule to index those keys on the server and improve query performance as described in Indexing Your Data.

Sort data

To retrieve sorted data, start by specifying one of the order-by methods to determine how results are ordered:

Method	Usage
queryOrderedByKey	Order results by child keys.
queryOrderedByValue	Order results by child values.
queryOrderedByChild	Order results by the value of a specified child key or nested child path.

You can only use one order-by method at a time. Calling an order-by method multiple times in the same query throws an error.The following example demonstrates how you could retrieve a list of a user's top posts sorted by their star count:

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// My top posts by number of stars
FIRDatabaseQuery *myTopPostsQuery = [[[self.ref child:@"user-posts"]
                                      child:[super getUid]]
                                     queryOrderedByChild:@"starCount"];This query retrieves the user's posts from the path in the database based on their user ID, ordered by the number of stars each post has received. This technique of using IDs as index keys is called data fan out, you can read more about it in Structure Your Database.The call to the 

queryOrderedByChild

 method specifies the child key to order the results by. In this example, posts are sorted by the value of the 

"starCount"

 child in each post. Queries can also be ordered by nested children, in case you have data that looks like this:"posts": {
  "ts-functions": {
    "metrics": {
      "views" : 1200000,
      "likes" : 251000,
      "shares": 1200,
    },
    "title" : "Why you should use TypeScript for writing Cloud Functions",
    "author": "Doug",
  },
  "android-arch-3": {
    "metrics": {
      "views" : 900000,
      "likes" : 117000,
      "shares": 144,
    },
    "title" : "Using Android Architecture Components with Firebase Realtime Database (Part 3)",
    "author": "Doug",
  }
},In this case, we can order our list elements by values nested under the 

metrics

 key by specifying the relative path to the nested child in our 

queryOrderedByChild

 call.

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FIRDatabaseQuery *postsByMostPopular = [[ref child:@"posts"] queryOrderedByChild:@"metrics/views"];For more information on how other data types are ordered, see How query data is ordered.

Filtering data

To filter data, you can combine any of the limit or range methods with an order-by method when constructing a query.

Method	Usage
queryLimitedToFirst	Sets the maximum number of items to return from the beginning of the ordered list of results.
queryLimitedToLast	Sets the maximum number of items to return from the end of the ordered list of results.
queryStartingAtValue	Return items greater than or equal to the specified key or value, depending on the order-by method chosen.
queryEndingAtValue	Return items less than or equal to the specified key or value, depending on the order-by method chosen.
queryEqualToValue	Return items equal to the specified key or value, depending on the order-by method chosen.

Unlike the order-by methods, you can combine multiple limit or range functions. For example, you can combine the 

queryStartingAtValue

 and 

queryEndingAtValue

 methods to limit the results to a specified range of values.

Limit the number of results

You can use the 

queryLimitedToFirst

 and 

queryLimitedToLast

 methods to set a maximum number of children to be synced for a given callback. For example, if you use 

queryLimitedToFirst

 to set a limit of 100, you initially only receive up to 100 

FIRDataEventTypeChildAdded

 callbacks. If you have fewer than 100 items stored in your Firebase database, an 

FIRDataEventTypeChildAdded

 callback fires for each item.As items change, you receive 

FIRDataEventTypeChildAdded

 callbacks for items that enter the query and 

FIRDataEventTypeChildRemoved

 callbacks for items that drop out of it so that the total number stays at 100.The following example demonstrates how an example blogging app might retrieve a list of the 100 most recent posts by all users:

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OBJECTIVE-C

// Last 100 posts, these are automatically the 100 most recent
// due to sorting by push() keys
FIRDatabaseQuery *recentPostsQuery = [[self.ref child:@"posts"] queryLimitedToFirst:100];

Filter by key or value

You can use 

queryStartingAtValue

, 

queryEndingAtValue

, and 

queryEqualToValue

 to choose arbitrary starting, ending, and equivalence points for queries. This can be useful for paginating data or finding items with children that have a specific value.

How query data is ordered

This section explains how data is sorted by each of the order-by methods in the 

FIRDatabaseQuery

class.

queryOrderedByKey

When using 

queryOrderedByKey

 to sort your data, data is returned in ascending order by key.Children with a key that can be parsed as a 32-bit integer come first, sorted in ascending order.
Children with a string value as their key come next, sorted lexicographically in ascending order.

queryOrderedByValue

When using 

queryOrderedByValue

, children are ordered by their value. The ordering criteria are the same as in 

queryOrderedByChild

, except the value of the node is used instead of the value of a specified child key.

queryOrderedByChild

When using 

queryOrderedByChild

, data that contains the specified child key is ordered as follows:Children with a 

nil

 value for the specified child key come first.
Children with a value of 

false

 for the specified child key come next. If multiple children have a value of 

false

, they are sorted lexicographically by key.
Children with a value of 

true

 for the specified child key come next. If multiple children have a value of 

true

, they are sorted lexicographically by key.
Children with a numeric value come next, sorted in ascending order. If multiple children have the same numerical value for the specified child node, they are sorted by key.
Strings come after numbers and are sorted lexicographically in ascending order. If multiple children have the same value for the specified child node, they are ordered lexicographically by key.
Objects come last and are sorted lexicographically by key in ascending order.

Detach listeners

Observers don't automatically stop syncing data when you leave a 

ViewController

. If an observer isn't properly removed, it continues to sync data to local memory. When an observer is no longer needed, remove it by passing the associated 

FIRDatabaseHandle

 to the 

removeObserverWithHandle

method.When you add a callback block to a reference, a 

FIRDatabaseHandle

 is returned. These handles can be used to remove the callback block.If multiple listeners have been added to a database reference, each listener is called when an event is raised. In order to stop syncing data at that location, you must remove all observers at a location by calling the 

removeAllObservers

 method.Calling 

removeObserverWithHandle

 or 

removeAllObservers

 on a listener does not automatically remove listeners registered on its child nodes; you must also be keep track of those references or handles to remove them.

Next Steps

Learn how to structure data
Store data offline