How to use the faunadb.query.Do function in faunadb

q.If(q.Equals(q.Select("ref", q.Var("buyer")), q.Select("ref", q.Var("seller"))),
            // Attempting to buy an item you are selling, removes it from sale
            q.Do(
              q.Update(q.Select("ref", q.Var("item")), {
                data : {
                  for_sale : false
                }
              }),
              "item removed from sale"
            ),
            // Check the credit balance of the purchasing player to ensure
            // they have available funds.
            q.If(q.LT(q.Var("buyerBalance"), q.Var("itemPrice")),
              "purchase failed: insufficient funds",
              // All clear! Preconditions passed.
              q.Do(
                // Write a purchase record.
                q.Create(q.Class("purchases"), {
                  data : {
                    item : q.Select("ref", q.Var("item")),
                    price : q.Var("itemPrice"),
                    buyer : q.Select("ref", q.Var("buyer")),
                    seller : q.Select("ref", q.Var("seller"))
                  }
                }),
                // Update the balances of the selling player and the purchasing player.
                q.Update(q.Select("ref", q.Var("buyer")), {
                  data : {
                    credits : q.Subtract(q.Var("buyerBalance"), q.Var("itemPrice"))
                  }
                }),
                q.Update(q.Select("ref", q.Var("seller")), {

// Load the current player and item based on their refs.
        buyer : q.Get(player.ref),
        item : q.Get(item.ref)
      }, q.Let({
        // Load the seller account and set the transaction price.
        isForSale : q.Select(["data", "for_sale"], q.Var("item")),
        itemPrice : q.Select(["data", "price"], q.Var("item")),
        buyerBalance : q.Select(["data", "credits"], q.Var("buyer")),
        seller : q.Get(q.Select(["data", "owner"], q.Var("item")))
      },
        // Check that the item is for sale.
        q.If(q.Not(q.Var("isForSale")),
          "purchase failed: item not for sale",
          q.If(q.Equals(q.Select("ref", q.Var("buyer")), q.Select("ref", q.Var("seller"))),
            // Attempting to buy an item you are selling, removes it from sale
            q.Do(
              q.Update(q.Select("ref", q.Var("item")), {
                data : {
                  for_sale : false
                }
              }),
              "item removed from sale"
            ),
            // Check the credit balance of the purchasing player to ensure
            // they have available funds.
            q.If(q.LT(q.Var("buyerBalance"), q.Var("itemPrice")),
              "purchase failed: insufficient funds",
              // All clear! Preconditions passed.
              q.Do(
                // Write a purchase record.
                q.Create(q.Class("purchases"), {
                  data : {