Moved a lot of functionality to mixins, improved parenting

app/imports/api/getModifierFields.js

@@ -0,0 +1,28 @@
+import MONGO_OPERATORS from '/imports/constants/MONGO_OPERATORS.js';
+
+const hasAny = function(values){
+  for (let value of values){
+    if (this.has(value)){
+      return true;
+    }
+  }
+  return false;
+};
+
+// Returns a Set of fields the modifier changes
+// The set has been extended with the "hasAny" function
+export default function getModifierFields (modifier) {
+  let fields = new Set();
+
+  for (let operator of MONGO_OPERATORS){
+    if (modifier[operator]) for (let field in modifier[operator]){
+      const indexOfDot = field.indexOf('.');
+      if (indexOfDot !== -1) {
+        field = field.substring(0, indexOfDot);
+      }
+      fields.add(field);
+    }
+  }
+  fields.hasAny = hasAny;
+  return fields;
+}