Add USD amount in FPMMTrade entity

Add a new field `collateralAmountUSD` as BigDecimal type to calculate
the value of `collateralAmount` in USD value at the moment of the trade.

Resolves: protofire#72

schema.graphql

@@ -199,6 +199,7 @@ type FpmmTrade @entity {
   creator: Account!
   creationTimestamp: BigInt!
   collateralAmount: BigInt!
+  collateralAmountUSD: BigDecimal!
   feeAmount: BigInt!
   outcomeIndex: BigInt!
   outcomeTokensTraded: BigInt!

src/FixedProductMarketMakerMapping.ts

@@ -35,7 +35,8 @@ function requireAccount(accountAddress: string): Account | null {
 }
 
 function recordTrade(fpmm: FixedProductMarketMaker, 
-    traderAddress: string, collateralAmount: BigInt,
+    traderAddress: string,
+    collateralAmount: BigInt, collateralAmountUSD: BigDecimal,
     feeAmount: BigInt, outcomeIndex: BigInt,
     outcomeTokensTraded: BigInt, tradeType: string,
     creationTimestamp: BigInt): void {
@@ -54,6 +55,7 @@ function recordTrade(fpmm: FixedProductMarketMaker,
     fpmmTrade.creator = traderAddress;
     fpmmTrade.creationTimestamp = creationTimestamp;
     fpmmTrade.collateralAmount = collateralAmount;
+    fpmmTrade.collateralAmountUSD = collateralAmountUSD;
     fpmmTrade.feeAmount = feeAmount;
     fpmmTrade.outcomeIndex = outcomeIndex;
     fpmmTrade.outcomeTokensTraded = outcomeTokensTraded;
@@ -272,6 +274,9 @@ export function handleBuy(event: FPMMBuy): void {
   let collateralUSDPrice = ethPerCollateral != null && usdPerEth != null ?
     ethPerCollateral.times(usdPerEth as BigDecimal) :
     zeroDec;
+  let collateralAmountUSD = (collateralUSDPrice != zeroDec) ?
+    collateralUSDPrice.times(event.params.investmentAmount.divDecimal(collateralScaleDec)) :
+    zeroDec;
 
   setLiquidity(fpmm as FixedProductMarketMaker, newAmounts, collateralScaleDec, collateralUSDPrice);
   increaseVolume(
@@ -290,7 +295,8 @@ export function handleBuy(event: FPMMBuy): void {
     event.params.buyer.toHexString());
 
   recordTrade(fpmm as FixedProductMarketMaker, 
-    event.params.buyer.toHexString(), event.params.investmentAmount,
+    event.params.buyer.toHexString(),
+    event.params.investmentAmount, collateralAmountUSD,
     event.params.feeAmount, event.params.outcomeIndex,
     event.params.outcomeTokensBought, TRADE_TYPE_BUY,
     event.block.timestamp);
@@ -325,6 +331,9 @@ export function handleSell(event: FPMMSell): void {
   let collateralUSDPrice = ethPerCollateral != null && usdPerEth != null ?
     ethPerCollateral.times(usdPerEth as BigDecimal) :
     zeroDec;
+  let collateralAmountUSD = (collateralUSDPrice != zeroDec) ?
+   collateralUSDPrice.times(event.params.returnAmount.divDecimal(collateralScaleDec)) :
+   zeroDec;
 
   setLiquidity(fpmm as FixedProductMarketMaker, newAmounts, collateralScaleDec, collateralUSDPrice);
   increaseVolume(
@@ -343,7 +352,8 @@ export function handleSell(event: FPMMSell): void {
     event.params.seller.toHexString());
 
   recordTrade(fpmm as FixedProductMarketMaker, 
-    event.params.seller.toHexString(), event.params.returnAmount,
+    event.params.seller.toHexString(),
+    event.params.returnAmount, collateralAmountUSD,
     event.params.feeAmount, event.params.outcomeIndex,
     event.params.outcomeTokensSold, TRADE_TYPE_SELL,
     event.block.timestamp);