Get default value from data file in Jekyll/Liquid

get.liquid

{%- comment -%}
  _includes/get.liquid
{%- endcomment %}

{%- comment -%}
  This file defines a Liquid function that outputs the value of a field
  in an object if it exists, and otherwise tries to retrieve a default value
  from a datafile. To put it differently, it retrieves a default value from
  a datafile, but allows you to override the defaults. 

  Suppose you have a datafile `data/people.yaml`:

    john:
      id: john
      first_name: John
      age: 20

    susan:
      id: susan
      first_name: Susan
      age: 40

  You reference this file in the front matter of a page :

    people: 
      - id: john
      - id: susan
        age: 20

  And then output the names and ages:

    {% for person in page.people %}
      {% include get.liquid object=person field="first_name" dataFile=site.data.people %}
      ({% include get.liquid object=person field="age" dataFile=site.data.people %})<br />
    {% endfor %}

  Output:

    John (20)
    Susan (20)

  For both, their first name was retrieved from the people.yaml file. The age
  of John was also retrieved there, but the age of Susan was overridden in 
  the front matter.

  License: MIT
  Author: Bas Cornelissen
  Date: 29 June 2021
{%- endcomment -%}

{%- comment -%}
  Parameters
{%- endcomment -%}
{%- assign dataFile = include.dataFile -%}
{%- assign object = include.object -%}
{%- assign field = include.field -%}

{%- comment -%}
  We use http://hamishwillee.github.io/2014/11/13/jekyll-includes-are-functions/
  First we check if the object is an array or a string. 
  We can do this using object.first: https://stackoverflow.com/questions/38917552/check-if-variable-is-type-of-string-or-array-in-liquid
  If it is an array, we define a defaultObject, which initially is just an empty
  array. To create such a thing in liquid you need to do strange things, see
  https://blog.rampatra.com/how-to-create-an-array-in-liquid
  Then we check if there exists an object in the data file, and if so we use
  that as a default object. We output the field, either from the object or the default object
{%- endcomment -%}

{%- comment -%}
  CASE 1: Object is an array
{%- endcomment -%}
{%- if object.first -%}

  {%- comment -%}
    If the object is a hash of the form `id: {id: ...}`,
    then object[0] is the key and object[1] is the actual
    object. We only look the latter (so you have 
    have to specify the id in the object!)
    https://stackoverflow.com/questions/51211556/check-if-variable-type-is-hash-or-array-in-liquid
  {%- endcomment -%}
  {%- unless object.first.first -%}
    {%- assign object = object[1] -%}
  {%- endunless -%}

  {%- comment -%}
    Get the default object: datafile[id], unless that does not exists,
    then the default is an empty object.
    (we create that in a hacky way: by splitting an empty string)
  {%- endcomment -%}
  {%- assign defaultObject = "" | split: "," -%}
  {%- if object contains "id" -%}
    {%- if dataFile contains object["id"] -%}
      {%- assign defaultObject = dataFile[object["id"]] -%}
    {%- endif -%}
  {%- endif -%}

  {%- comment -%}
    Output the value: object[field] or otherwise defaultObject[field] 
    if the former does not exist
  {%- endcomment -%}
  {%- if object contains field -%}
    {{- object[field] -}}
  {%- elsif defaultObject contains field -%}
    {{- defaultObject[field] -}}
  {%- endif -%}

{%- comment -%}
  CASE 2: object is not an array, but a string
{%- endcomment -%}
{%- else -%}
  {%- comment -%}
    If the object exists in the datafile, try to output 
  {%- endcomment -%}
  {%- if dataFile contains object -%}
    {%- if dataFile[object] contains field -%}
      {{- dataFile[object][field] -}}
    {%- endif -%}

  {%- comment -%}
    object does not exist, output raw text only if field = 'fulltext'
  {%- endcomment -%}
  {%- elsif field == "fulltext" -%}
    {{- object -}}
  {%- endif -%}
{%- endif -%}

mypage.html

---
people:
  - id: john
  - id: susan
    age: 20
---

{%- for person in page.people %}
  {% include get.liquid object=person field="first_name" dataFile=site.data.people %}
  ({% include get.liquid object=person field="age" dataFile=site.data.people %}) <br />
{%- endfor %}

output.html

John (20) <br />
Susan (20) <br />

people.yaml

# _data/people.yaml

john:
  id: john
  first_name: John
  age: 20

susan:
  id: susan
  first_name: Susan
  age: 40