Provide additional context on JSON field error

Author: josevalim

integration_test/pg/exceptions_test.exs

@@ -0,0 +1,13 @@
+defmodule Ecto.Integration.ExceptionsTest do
+  use Ecto.Integration.Case, async: true
+
+  alias Ecto.Integration.TestRepo
+  alias Ecto.Integration.Post
+  import Ecto.Query, only: [from: 2]
+
+  test "on bad JSON interpolation" do
+    assert_raise Postgrex.Error,
+                 ~r/If you are trying to query a JSON field, the parameter must be interpolated/,
+                 fn -> TestRepo.all(from p in Post, where: p.meta["field"] == "example") end
+  end
+end

lib/ecto/adapters/postgres/connection.ex

@@ -67,7 +67,23 @@ if Code.ensure_loaded?(Postgrex) do
 
     @impl true
     def prepare_execute(conn, name, sql, params, opts) do
-      Postgrex.prepare_execute(conn, name, sql, params, opts)
+      case Postgrex.prepare_execute(conn, name, sql, params, opts) do
+        {:error, %Postgrex.Error{postgres: %{pg_code: "22P02", message: message}} = error} ->
+          context = """
+          . If you are trying to query a JSON field, the parameter must be interpolated. Instead of
+
+              p.json["field"] == "value"
+
+          do
+
+              p.json["field"] == ^"value"
+          """
+
+          {:error, put_in(error.postgres.message, message <> context)}
+        other ->
+          other
+        end
+
     end
 
     @impl true