接口缺少方法的实现

Came across a tutorial about organizing db access in Go: https://www.alexedwards.net/blog/organising-database-access

I'm following the "Use an interface" approach but bump into an error basically saying I haven't implemented any methods for an interface while I have...

In users.go:

package fs

import (
...
)

func (client *Client) Example_Write() error {
    ...
    return nil
}

func (client *Client) Example_Query() error {
    ...
    return nil
}

In db.go:

type Datastore interface {
    Example_Write() error
    Example_Query() error
}

type Client struct {
    *firestore.Client
}

func SetupClient() (*Client, error){
    return &Client{client}, nil
}
...

In app.go:

import ( "fs" )
type Env struct {
    client fs.Datastore
}

client, _ := fs.SetupClient()
env := &Env{client: client}

Error looks like this: 188:14: cannot use client (type *fs.Client) as type fs.Datastore in field value: *fs.Client does not implement fs.Datastore (missing Example_Query method)

Any idea would be appreciated :) Very new to Go so, please bear with me!