Common mistakes

-- Wrong: the first one may still go through, so the player can get paid twice
local Ok, Why = PlayerStore:Edit(UserId, AddGold)
if Why == "Unresolved" then
	PlayerStore:Edit(UserId, AddGold)
end

-- Right: leave it. Ledger keeps sending the first one until it knows.
local Ok, Why = PlayerStore:Edit(UserId, AddGold)
if Why == "Unresolved" then
	return
end

-- Wrong: a read that failed looks like a brand new player
local Data = PlayerStore:Peek(UserId)
local Gold = if Data then Data.Gold else 0

-- Right: only Missing means "nothing saved"
local Data, Why = PlayerStore:Peek(UserId)
if not Data and Why ~= "Missing" then
	return -- couldn't read it, try again later
end
local Gold = if Data then Data.Gold else 0

-- Wrong: changes the data Ledger handed you, not a copy
if Op.Kind == "AddGold" then
	Data.Gold += Op.Amount
	return Data
end

-- Right: change a copy
if Op.Kind == "AddGold" then
	local New = table.clone(Data)
	New.Gold += Op.Amount
	return New
end

-- Wrong: the reducer can run more than once, and give a different answer each time
if Op.Kind == "OpenEgg" then
	local Pet = PETS[math.random(#PETS)]
	-- ...
end

-- Right: pick it outside, and put it in the op
local Pet = PETS[math.random(#PETS)]
Session:Apply({ Kind = "OpenEgg", Pet = Pet })

-- Wrong: the player sees gold that isn't saved
Player.leaderstats.Gold.Value += 10
PlayerStore:Edit(Player.UserId, AddGold)

-- Right: change the data with an op, and show what was saved
local Ok = PlayerStore:Edit(Player.UserId, AddGold)
local Session = PlayerStore:Get(Player)
if Ok and Session then
	Session:Refresh()
end

-- Wrong: the session still shows the old gold until its next save
PlayerStore:Edit(Player.UserId, AddGold)
print(Session:Get().Gold)

-- Right: Refresh once it's saved
local Ok = PlayerStore:Edit(Player.UserId, AddGold)
if Ok then
	Session:Refresh()
end

-- Wrong: the player just spent gold with Apply, but it isn't saved yet,
-- so the trade sees the gold they no longer have
Session:Apply({ Kind = "SpendGold", Amount = 100 })
Ledger.Tx({ ... })

-- Right: save what's waiting first
Session:Apply({ Kind = "SpendGold", Amount = 100 })
Session:Flush()
Ledger.Tx({ ... })

-- Wrong: a second Ledger.New for the same store, in another script
local PlayerStore = Ledger.New<<PlayerData, PlayerOps>>({ Name = "PlayerData", ... })

-- Right: make it once in Stores, and require it everywhere
local Stores = require(ServerStorage.Stores)
local PlayerStore = Stores.PlayerStore

-- Wrong: players who saved before this update have no Pets, so Data.Pets is nil
Default = { Gold = 0, Pets = {} },

-- Right: add the field to Default and add a migration
Default = { Gold = 0, Pets = {} },
Migrations = {
	{
		Fields = { "Pets" },
		Run = function(Stored: unknown): unknown
			local New = table.clone(Stored :: Saved)
			New.Pets = {}
			return New
		end,
	},
},

-- Wrong: Bump can take up to a minute, and the player waits for it
StatsStore:Bump("BossKills", 1)

-- Right: run it in the background
task.spawn(function()
	StatsStore:Bump("BossKills", 1)
end)

-- Wrong: once the sale is closed, Take answers Missing again, and this makes the stock again
if Result == "Missing" then
	Stock:Open()
end

-- Right: only open while the sale is on
if Result == "Missing" and os.time() < SALE_ENDS then
	Stock:Open()
end

-- Wrong: Edit waits, and a Follow function can't
GuildStore:Follow(GuildId):Subscribe(function(Value)
	GuildStore:Edit(GuildId, Op)
end)

-- Right: start it in a new thread
GuildStore:Follow(GuildId):Subscribe(function(Value)
	task.spawn(function()
		GuildStore:Edit(GuildId, Op)
	end)
end)

-- Wrong: by the time this runs, Ledger is already closing, and Apply answers Closed
game:BindToClose(function()
	PayOutRounds()
end)

-- Right
Ledger.BeforeClose(function()
	PayOutRounds()
end)