List posts

In this chapter, you will develop a feature that enables users to retrieve all of the blog posts stored on your blockchain application. The feature will allow users to perform a query and receive a paginated response, which means that the output will be divided into smaller chunks or "pages" of data. This will allow users to more easily navigate and browse through the list of posts, as they will be able to view a specific number of posts at a time rather than having to scroll through a potentially lengthy list all at once.

List posts

Let's implement the ListPost keeper method that will be called when a user makes a query to the blockchain application, requesting a paginated list of all the posts stored on chain.

x/blog/keeper/query_list_post.go

package keeper

import (
    "context"

    "blog/x/blog/types"

    "github.com/cosmos/cosmos-sdk/store/prefix"
    sdk "github.com/cosmos/cosmos-sdk/types"
    "github.com/cosmos/cosmos-sdk/types/query"
    "google.golang.org/grpc/codes"
    "google.golang.org/grpc/status"
)

func (k Keeper) ListPost(goCtx context.Context, req *types.QueryListPostRequest) (*types.QueryListPostResponse, error) {
    if req == nil {
        return nil, status.Error(codes.InvalidArgument, "invalid request")
    }

    var posts []types.Post
    ctx := sdk.UnwrapSDKContext(goCtx)

    store := ctx.KVStore(k.storeKey)
    postStore := prefix.NewStore(store, types.KeyPrefix(types.PostKey))

    pageRes, err := query.Paginate(postStore, req.Pagination, func(key []byte, value []byte) error {
        var post types.Post
        if err := k.cdc.Unmarshal(value, &post); err != nil {
            return err
        }

        posts = append(posts, post)
        return nil
    })

    if err != nil {
        return nil, status.Error(codes.Internal, err.Error())
    }

    return &types.QueryListPostResponse{Post: posts, Pagination: pageRes}, nil
}

ListPost takes in two arguments: a context object and a request object of type QueryListPostRequest. It returns a response object of type QueryListPostResponse and an error.

The function first checks if the request object is nil and returns an error with a InvalidArgument code if it is. It then initializes an empty slice of Post objects and unwraps the context object.

It retrieves a key-value store from the context using the storeKey field of the keeper struct and creates a new store using a prefix of the PostKey. It then calls the Paginate function from the query package on the store and the pagination information in the request object. The function passed as an argument to Paginate iterates over the key-value pairs in the store and unmarshals the values into Post objects, which are then appended to the posts slice.

If an error occurs during pagination, the function returns an Internal error with the error message. Otherwise, it returns a QueryListPostResponse object with the list of posts and pagination information.

Modify QueryListPostResponse

Add a repeated keyword to return a list of posts and include the option [(gogoproto.nullable) = false] to generate the field without a pointer.

proto/blog/blog/query.proto

message QueryListPostResponse {
  repeated Post post = 1 [(gogoproto.nullable) = false];
  cosmos.base.query.v1beta1.PageResponse pagination = 2;
}

Run the command to generate Go files from proto:

ignite generate proto-go