Verify slug field exists on config load.

src/backends/backend.js

@@ -2,6 +2,7 @@ import { attempt, isError } from 'lodash';
 import { resolveFormat } from "Formats/formats";
 import { selectIntegration } from 'Reducers/integrations';
 import {
+  selectIdentifier,
   selectListMethod,
   selectEntrySlug,
   selectEntryPath,
@@ -45,15 +46,10 @@ const slugFormatter = (template = "{{slug}}", entryData) => {
   const date = new Date();
 
   const getIdentifier = (entryData) => {
-    const validIdentifierFields = ["title", "path"];
-    const identifiers = validIdentifierFields.map((field) =>
-      entryData.find((_, key) => key.toLowerCase().trim() === field)
-    );
-
-    const identifier = identifiers.find(ident => ident !== undefined);
+    const identifier = selectIdentifier(entryData.keys());
 
     if (identifier === undefined) {
-      throw new Error(`Collection must have a field name that is a valid entry identifier. Please add one of these fields: ${ validIdentifierFields }`);
+      throw new Error("Collection must have a field name that is a valid entry identifier");
     }
 
     return identifier;

src/reducers/collections.js

@@ -34,7 +34,8 @@ function validateCollection(configCollection) {
     files,
     format,
     extension,
-    frontmatter_delimiter: delimiter
+    frontmatter_delimiter: delimiter,
+    fields,
   } = configCollection.toJS();
 
   if (!folder && !files) {
@@ -51,8 +52,22 @@ function validateCollection(configCollection) {
     // Cannot set custom delimiter without explicit and proper frontmatter format declaration
     throw new Error(`Please set a proper frontmatter format for collection "${name}" to use a custom delimiter. Supported frontmatter formats are yaml-frontmatter, toml-frontmatter, and json-frontmatter.`);
   }
+  if (selectIdentifier(fields.map(f => f.name)) === undefined) {
+    // Verify that the collection has a slug field.
+    // TODO: Verify only for folder-type collections.
+    throw new Error(`Collection "${name}" must have a field that is a valid entry identifier. Supported fields are ${validIdentifierFields.join(',')}`);
+  }
 }
 
+const validIdentifierFields = ["title", "path"];
+export const selectIdentifier = (entryData, collectionName) => {
+  const identifiers = validIdentifierFields.map((field) =>
+    entryData.find(key => key.toLowerCase().trim() === field)
+  );
+
+  return identifiers.find(ident => ident !== undefined);
+};
+
 const selectors = {
   [FOLDER]: {
     entryExtension(collection) {