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Paginating data causing unusual behavior?

I am displaying “global posts” on one of my tabs. Currently, there are only 11 posts in the database:

enter image description here

In the app Some of the posts are being duplicated, and I have no idea why these SPECIFIC posts are being duplicated, as it seems to me like it is happening at random.

Here is the code for how I paginate the data.

  1. When the component mounts, I query firestore and pull 5 posts using getCollection().

.

async componentDidMount() {
   
 this.unsubscribe = Firebase.firestore()
.collection('globalPosts')
.orderBy("date_created", "desc")
.limit(5)
.onSnapshot(this.getCollection);  

}
  1. I get the posts successfully in getCollection(), and set an index, lastItemIndex, so I know where to query for the next posts

.

getCollection = (querySnapshot) => {
        const globalPostsArray = [];
        querySnapshot.forEach((res) => {
        const { 
            ..fields
            } = res.data();

            globalPostsArray.push({
                ..fields
            });
        });


        this.setState({
            globalPostsArray,
            isLoading: false, 
            lastItemIndex: globalPostsArray.length - 1
        });
}
  1. This gets the first 5 items, no problem, ordered by date_created, descending.

If the user scrolls down the flatlist, I have logic in the flatlist to handle fetching more data:

            <FlatList
                data={this.state.globalPostsArray}
                renderItem={renderItem}
                keyExtractor={item => item.key}
                contentContainerStyle={{ paddingBottom: 50 }}
                showsHorizontalScrollIndicator={false}
                showsVerticalScrollIndicator={false}
                onRefresh={this._refresh}
                refreshing={this.state.isLoading}
                onEndReachedThreshold={0.5} <---------------------- Threshold
                onEndReached={() => {this.getMore()}} <------------ Get more data
            />
  1. Finally, once it is time to retrieve more data, I call this.getMore()

Here is the code to get the next 5 posts:

getMore = async() => {
    const newPostsArray = [] <-------- new array for the next 5 posts

    Firebase.firestore()
    .collection('globalPosts')
    .orderBy("date_created", "desc")
    .startAfter(this.state.globalPostsArray[this.state.lastItemIndex].date_created) <--- note start after
    .limit(5)
    .onSnapshot(querySnapshot => {
        querySnapshot.forEach((res) => {
            const { 
                ... same fields as getCollection()
                } = res.data();

                newPostsArray.push({
                    ... same fields as getCollection()
                });
            });

            
            this.setState({
                globalPostsArray: this.state.globalPostsArray.concat(newPostsArray), <--- add to state array
                lastItemIndex: this.state.globalPostsArray.length-1 <---- increment index
            });

            console.log(this.state.lastItemIndex) <------- I print out last item index
    })
}

Some notes:

  1. The code works fine in terms of fetching the data

  2. The code works fine in terms of pagination, and only fetches 5 posts at a time

  3. There is no discernible pattern I am seeing in which posts are being duplicated

  4. I am ordering by date_created, descending when querying firestore in both getCollection() and getMore()

  5. I console log “last item index” in my getMore(), and of course the index is higher than the number of posts

  6. I keep getting the following warning/error, with different keys (post ID’s in firestore), which shows me the duplication is happening at random, and not specific to one user. This warning/error doesn’t break the application, but is telling me this weird behavior is happening:

    Encountered two children with the same key, ZJu3FbhzOkXDM5mn6O6T. Keys should be unique so that components maintain their identity across updates. Non-unique keys may cause children to be duplicated and/or omitted — the behavior is unsupported and could change in a future version.

Can someone point me in the right direction, why my pagination is having such unusual behavior?

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Answer

My issue was with lastItemIndex. Saving it in state was causing problems. I solved the problem by removing lastItemIndex from state, and making it a local variable in getMore():

getMore = async() => {
    const newPostsArray = []

    const lastItemIndex = this.state.globalPostsArray.length - 1 <---- added it here

    await Firebase.firestore()
    .collection('globalPosts')
    .orderBy("date_created", "desc")
    .startAfter(this.state.globalPostsArray[lastItemIndex].date_created)
    .limit(5)
    .onSnapshot(querySnapshot => {
        querySnapshot.forEach((res) => {
            const { 
                ..fields
                } = res.data();

                newPostsArray.push({
                    key: res.id,
                    ..fields
                });
            });

            
            this.setState({
                globalPostsArray: this.state.globalPostsArray.concat(newPostsArray)
            });

    })
}
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