Use lookup instead of filtering the list to get a symbol’s exchange rate

apps/explorer/lib/explorer/exchange_rates/exchange_rates.ex

@@ -90,6 +90,19 @@ defmodule Explorer.ExchangeRates do
     list_from_store(store())
   end
 
+  @doc """
+  Returns a specific rate from the tracked tickers by symbol
+  """
+  @spec lookup(String.t()) :: Token.t()
+  def lookup(symbol) do
+    if store() == :ets do
+      case :ets.lookup(table_name(), symbol) do
+        [{_key, token} | _] -> token
+        _ -> nil
+      end
+    end
+  end
+
   ## Undocumented public functions
 
   @doc false

apps/explorer/lib/explorer/market/market.ex

@@ -12,10 +12,9 @@ defmodule Explorer.Market do
   @doc """
   Get most recent exchange rate for the given symbol.
   """
-  @spec fetch_exchange_rate(String.t()) :: Token.t()
+  @spec get_exchange_rate(String.t()) :: Token.t()
-  def fetch_exchange_rate(symbol) do
+  def get_exchange_rate(symbol) do
-    ExchangeRates.list()
+    ExchangeRates.lookup(symbol)
-    |> Enum.find(fn token -> token.symbol == symbol end)
   end
 
   @doc """

apps/explorer/test/explorer/exchange_rates/exchange_rates_test.exs

@@ -97,13 +97,26 @@ defmodule Explorer.ExchangeRatesTest do
     ExchangeRates.init([])
 
     rates = [
-      %Token{id: "z", symbol: "z"},
+      %Token{symbol: "z"},
-      %Token{id: "a", symbol: "a"}
+      %Token{symbol: "a"}
     ]
 
     expected_rates = Enum.reverse(rates)
-    for rate <- rates, do: :ets.insert(ExchangeRates.table_name(), {rate.id, rate})
+    for rate <- rates, do: :ets.insert(ExchangeRates.table_name(), {rate.symbol, rate})
 
     assert expected_rates == ExchangeRates.list()
   end
+
+  test "lookup/1" do
+    ExchangeRates.init([])
+
+    z = %Token{symbol: "z"}
+
+    rates = [z, %Token{symbol: "a"}]
+
+    for rate <- rates, do: :ets.insert(ExchangeRates.table_name(), {rate.symbol, rate})
+
+    assert z == ExchangeRates.lookup("z")
+    assert nil == ExchangeRates.lookup("nope")
+  end
 end

apps/explorer/test/explorer/market/market_test.exs

@@ -1,31 +1,10 @@
 defmodule Explorer.MarketTest do
   use Explorer.DataCase
 
-  alias Explorer.ExchangeRates
-  alias Explorer.ExchangeRates.Token
   alias Explorer.Market
   alias Explorer.Market.MarketHistory
   alias Explorer.Repo
 
-  describe "fetch_exchange_rate/1" do
-    setup do
-      use_ets_store()
-
-      {:ok, _} = ExchangeRates.start_link([])
-      rate = %Token{id: "POA", symbol: "POA"}
-      :ets.insert(ExchangeRates.table_name(), {rate.id, rate})
-      {:ok, %{rate: rate}}
-    end
-
-    test "with matching symbol", %{rate: rate} do
-      assert Market.fetch_exchange_rate("POA") == rate
-    end
-
-    test "with no matching symbol" do
-      assert Market.fetch_exchange_rate("ETH") == nil
-    end
-  end
-
   test "fetch_recent_history/1" do
     today = Date.utc_today()
 
@@ -105,20 +84,4 @@ defmodule Explorer.MarketTest do
       assert fetched_record.opening_price == new_record.opening_price
     end
   end
-
-  defp use_ets_store do
-    # Use ets tables as ExchangeRates store and put some test data in to
-    # exercise Context filtering
-    exchange_config = Application.get_env(:explorer, Explorer.ExchangeRates)
-
-    Application.put_env(
-      :explorer,
-      Explorer.ExchangeRates,
-      Keyword.put(exchange_config, :store, :ets)
-    )
-
-    on_exit(fn ->
-      Application.put_env(:explorer, Explorer.ExchangeRates, exchange_config)
-    end)
-  end
 end

apps/explorer_web/lib/explorer_web/controllers/chain_controller.ex

@@ -12,7 +12,7 @@ defmodule ExplorerWeb.ChainController do
       "show.html",
       chain: Statistics.fetch(),
       market_history_data: Market.fetch_recent_history(30),
-      exchange_rate: Market.fetch_exchange_rate(coin()) || Token.null()
+      exchange_rate: Market.get_exchange_rate(coin()) || Token.null()
     )
   end