Handling Developer Products

The following code is an example of how you would handle players purchasing developer products:

local DEFAULT_DATA = { coins = 100, recentPurchases = {} }
local RECENT_PURCHASES_LIMIT = 100
local PRODUCTS = {
    [12345] = function(oldData)
        -- Product callbacks return a new version of the data.
        return Sift.Dictionary.merge(oldData, {
            coins = oldData.coins + 100,
        })
    end,
}

local collection = Lapis.createCollection("PlayerData", {
    defaultData = DEFAULT_DATA,
    validate = t.strictInterface({ coins = t.integer, recentPurchases = t.array(t.string) }),
})

local documents = {}

local function onPlayerAdded(player: Player)
    collection
        :load(`Player{player.UserId}`, { player.UserId })
        :andThen(function(document)
            if player.Parent == nil then
                document:close():catch(warn)
                return
            end

            documents[player] = document
        end)
        :catch(function(message)
            warn(`Player {player.Name}'s data failed to load: {message}`)
            player:Kick("Data failed to load.")
        end)
end

local function onPlayerRemoving(player: Player)
    local document = documents[player]

    if document ~= nil then
        documents[player] = nil
        document:close():catch(warn)
    end
end

local function processReceipt(receiptInfo): Enum.ProductPurchaseDecision
    local player = Players:GetPlayerByUserId(receiptInfo.PlayerId)
    if player == nil then
        return Enum.ProductPurchaseDecision.NotProcessedYet
    end

    while documents[player] == nil and player.Parent ~= nil do
        -- Wait until the document loads or the player leaves.
        task.wait()
    end

    local document = documents[player]
    if document == nil then
        return Enum.ProductPurchaseDecision.NotProcessedYet
    end

    local data = document:read()

    if table.find(data.recentPurchases, receiptInfo.PurchaseId) then
        -- The purchase has been added to the player's data, but it might not have saved yet.
        local saveOk = document:save():await()

        if saveOk then
            return Enum.ProductPurchaseDecision.PurchaseGranted
        else
            return Enum.ProductPurchaseDecision.NotProcessedYet
        end
    end

    -- The product callback must not yield. Otherwise, it can return outdated data and overwrite new changes.
    local productOk, dataWithProduct = pcall(PRODUCTS[receiptInfo.ProductId], data)
    if not productOk then
        return Enum.ProductPurchaseDecision.NotProcessedYet
    end

    local newRecentPurchases = Sift.Array.push(data.recentPurchases, receiptInfo.PurchaseId)
    if #newRecentPurchases > RECENT_PURCHASES_LIMIT then
        newRecentPurchases = Sift.Array.shift(newRecentPurchases, #newRecentPurchases - RECENT_PURCHASES_LIMIT)
    end

    document:write(Sift.Dictionary.merge(dataWithProduct, {
        recentPurchases = newRecentPurchases,
    }))

    local saveOk = document:save():await()
    if not saveOk then
        return Enum.ProductPurchaseDecision.NotProcessedYet
    end

    return Enum.ProductPurchaseDecision.PurchaseGranted
end

-- The ProcessReceipt callback must be set before the Players.PlayerAdded signal is fired, otherwise the player's
-- existing receipts won't be processed when they join.
MarketplaceService.ProcessReceipt = processReceipt

Players.PlayerAdded:Connect(onPlayerAdded)
Players.PlayerRemoving:Connect(onPlayerRemoving)

for _, player in Players:GetPlayers() do
    onPlayerAdded(player)
end