Getting duplicate elements in react component after fetchMore

Question

I want to fetchMore data when onscroll reaches the bottom. The issue I have is that I get duplicate elements thus console yelling at me as well that I nee unique element keys. I'm getting the correct data in the graphql playground so it's an issue in my react component. Does this logic make sense? What's going wrong here?

class App extends Component {
  isBottom = (fetchMore, data) => {
    window.onscroll = () => {
      if (
        data &&
        data.infiniteScrollMovies &&
        data.infiniteScrollMovies.hasMore &&
        window.innerHeight + document.documentElement.scrollTop === document.documentElement.offsetHeight
      ) {
        console.log('YES!')
        this.fetchMoreData(fetchMore, data)
      }
    }
  }

  fetchMoreData = (fetchMore, data) => {
    fetchMore({
      variables: {
        offset: data.infiniteScrollMovies.newOffset,
      },
      updateQuery: (prev, { fetchMoreResult }) => {
        if (!fetchMoreResult) return prev

        return {
          ...fetchMoreResult,
          infiniteScrollMovies: {
            ...fetchMoreResult.infiniteScrollMovies,
            movies: [...prev.infiniteScrollMovies.movies, ...fetchMoreResult.infiniteScrollMovies.movies],
          },
        }
      },
    })
  }

  render() {
    return (
      <div className="flex flex-column">
        <Query query={ALL_MOVIES} notifyOnNetworkStatusChange={true} fetchPolicy="network-only">
          {({ data, loading, error, fetchMore }) => {
            if (error) return <h1>{error.message}</h1>
            this.isBottom(fetchMore, data)
            return (
              <div>
                {data &&
                  data.infiniteScrollMovies &&
                  data.infiniteScrollMovies.movies.map(m => (
                    <article key={m.id}>
                    // ... rendering an element
                    </article>
                  ))}
           <div>{loading && <h1 className="red text-center">Loading...</h1>}</div>

Here's my query:

export const ALL_MOVIES = gql`
  query infiniteScrollMovies($offset: Int) {
    infiniteScrollMovies(offset: $offset, limit: 16) {
      movies {
        image_url
        title
        id
      }
      hasMore
      newOffset
    }
  }
`

Answer 1

Use the variables parameter in updataeQuery to handle this scenario, basically you need to prevent the appending in cache if already its done.

Change your updateQuery to below

        updateQuery: (prev, { variables, fetchMoreResult }) => {
// Return the prev values, if it allready has the length specified by offset
// thereby preventing duplicate values in cache.
                        if (!fetchMoreResult || 
                           prev.infiniteScrollMovies.movies.length > variables.offset) 
                         return prev