Progress on file system UI

app/imports/api/creature/creatures/methods/assertHasCharacterSlots.js

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+import { getUserTier } from '/imports/api/users/patreon/tiers.js';
+import Creatures from '/imports/api/creature/creatures/Creatures.js';
+
+export default function assertHasCharactersSlots(userId) {
+  if (characterSlotsRemaining(userId) <= 0) {
+    throw new Meteor.Error('characterSlotLimit',
+      `You are already at your limit of ${tier.characterSlots} characters`)
+  }
+}
+
+export function characterSlotsRemaining(userId) {
+  let tier = getUserTier(userId);
+  const currentCharacterCount = Creatures.find({
+    owner: userId,
+  }, {
+    fields: { _id: 1 },
+  }).count();
+  if (tier.characterSlots === -1) {
+    return Number.POSITIVE_INFINITY;
+  }
+  return tier.characterSlots - currentCharacterCount;
+}