Updating posts

In this chapter, we will be focusing on the process of handling an "update post" message.

To update a post, you need to retrieve the specific post from the store using the "Get" operation, modify the values, and then write the updated post back to the store using the "Set" operation.

Let's first implement a getter and a setter logic.

Getting posts

Implement the GetPost keeper method in post.go:

x/blog/keeper/post.go

func (k Keeper) GetPost(ctx sdk.Context, id uint64) (val types.Post, found bool) {
    store := prefix.NewStore(ctx.KVStore(k.storeKey), types.KeyPrefix(types.PostKey))
    b := store.Get(GetPostIDBytes(id))
    if b == nil {
        return val, false
    }
    k.cdc.MustUnmarshal(b, &val)
    return val, true
}

GetPost takes in two arguments: a context ctx and an id of type uint64 representing the ID of the post to be retrieved. It returns a types.Post struct containing the values of the post, and a boolean value indicating whether the post was found in the database.

The function first creates a store using the prefix.NewStore method, passing in the key-value store from the context and the types.KeyPrefix function applied to the types.PostKey constant as arguments. It then attempts to retrieve the post from the store using the store.Get method, passing in the ID of the post as a byte slice. If the post is not found in the store, it returns an empty types.Post struct and a boolean value of false.

If the post is found in the store, the function unmarshals the retrieved byte slice into a types.Post struct using the cdc.MustUnmarshal method, passing in a pointer to the val variable as an argument. It then returns the val struct and a boolean value of true to indicate that the post was found in the database.

Setting posts

Implement the SetPost keeper method in post.go:

x/blog/keeper/post.go

func (k Keeper) SetPost(ctx sdk.Context, post types.Post) {
    store := prefix.NewStore(ctx.KVStore(k.storeKey), types.KeyPrefix(types.PostKey))
    b := k.cdc.MustMarshal(&post)
    store.Set(GetPostIDBytes(post.Id), b)
}

SetPost takes in two arguments: a context ctx and a types.Post struct containing the updated values for the post. The function does not return anything.

The function first creates a store using the prefix.NewStore method, passing in the key-value store from the context and the types.KeyPrefix function applied to the types.PostKey constant as arguments. It then marshals the updated post struct into a byte slice using the cdc.MustMarshal method, passing in a pointer to the post struct as an argument. Finally, it updates the post in the store using the store.Set method, passing in the ID of the post as a byte slice and the marshaled post struct as arguments.

Update posts

x/blog/keeper/msg_server_update_post.go

package keeper

import (
    "context"
    "fmt"

    "blog/x/blog/types"

    sdk "github.com/cosmos/cosmos-sdk/types"
    sdkerrors "github.com/cosmos/cosmos-sdk/types/errors"
)

func (k msgServer) UpdatePost(goCtx context.Context, msg *types.MsgUpdatePost) (*types.MsgUpdatePostResponse, error) {
    ctx := sdk.UnwrapSDKContext(goCtx)
    var post = types.Post{
        Creator: msg.Creator,
        Id:      msg.Id,
        Title:   msg.Title,
        Body:    msg.Body,
    }
    val, found := k.GetPost(ctx, msg.Id)
    if !found {
        return nil, sdkerrors.Wrap(sdkerrors.ErrKeyNotFound, fmt.Sprintf("key %d doesn't exist", msg.Id))
    }
    if msg.Creator != val.Creator {
        return nil, sdkerrors.Wrap(sdkerrors.ErrUnauthorized, "incorrect owner")
    }
    k.SetPost(ctx, post)
    return &types.MsgUpdatePostResponse{}, nil
}

UpdatePost takes in a context and a message MsgUpdatePost as input, and returns a response MsgUpdatePostResponse and an error. The function first retrieves the current values of the post from the database using the provided msg.Id, and checks if the post exists and if the msg.Creator is the same as the current owner of the post. If either of these checks fail, it returns an error. If both checks pass, it updates the post in the database with the new values provided in msg, and returns a response without an error.

Summary

Well done! You have successfully implemented a number of important methods for managing posts within a store.

The GetPost method allows you to retrieve a specific post from the store based on its unique identification number, or post ID. This can be useful for displaying a specific post to a user, or for updating it.

The SetPost method enables you to update an existing post in the store. This can be useful for correcting mistakes or updating the content of a post as new information becomes available.

Finally, you implemented the UpdatePost method, which is called whenever the blockchain processes a message requesting an update to a post.