Hugo and Data: Advanced Transformations

written by Regis Philibert
9 minute read
Article :Hugo and Data: Advanced Transformations

Transform!

Last time, using famous gents from Britain we covered some data manipulation, but it implied hardcoding of lot of gent’s info.

Usually your data comes from a source you lack control of like an API, or a data file or most usually a user managed content file. In this new article about Hugo and Data, we’ll cover how you can take data from a limited source (basic Front Matters, API endpoints) and transform it into objects better suited to your project’s needs! We’ll use a “transformer” partial and even some remote data fetching to complement our gent!

If you’re stumbling on this article without having read through this one, I strongly suggest you go back to it as it covers the basics of two very important data types which we’ll use a lot here: Slice and Maps.

For this illustrative context, our data source will be markdown files and our gents will be structured this way:

---
# content/gent/john-lennon.md
title: John Lennon
date: 1940-10-09
bands:
- Beatles
instruments:
  - Piano
  - Guitar
  - Vocals
---

John Lennon was an English singer, songwriter, musician and peace activist who achieved worldwide fame as the founder...

Up there is our source, our input. We’ll want to transform it into this data object:

"fullname" String
"firstname" String
"lastname" String
"birthdate" Date
"city" String
"age" Int
"instruments" Map
  "number" int
  "string_rep" String
  "list" Slice
"songs" Slice

Transform with a range

Let’s range on our gents and create a new slice with the transformed entries:

{{ $new_gents := slice }}
{{ range site.RegularPages ".Type" "gent" }}
  {{ $new_gents = $new_gents | append (dict
    "fullname" .Title
    "firstname" (index 0 (split .Title " "))
    "lastname" (index 1 (split .Title " "))
    "birthdate" .Date
    "city" "Liverpool"
    "instruments" (dict
      "list" .instruments
      "string_rep" (delimit .instruments ", " "and")
      "number" (len .instruments)

    )
  )
  }}
{{ end }}

Name

"fullname" .Title
"firstname" (index 0 (split .Title " "))
"lastname" (index 1 (split .Title " "))

First thing of note is that we only have a fullname under the file’s .Title. We add it as fullname because it makes much more sense.

If you’ve followed the first part well you should be able to see what’s happening next.
We’re using split a function which does the opposite of delimit. It takes a string as first parameter and creates a slice with all the substrings delimited by the second parameter — here a whitespace between our two words.
Of course, the firstname will be the first entry at 0 and lastname the second one at 1. We use index to retrieve those.

Birthdate

"birthdate" .Date

We also pass the entry’s .Date as birthdate.

City

"city" "Liverpool"

We know the city is always “Liverpool”!

Instruments

"instruments" (dict
  "list" .instruments
  "string_rep" (delimit .instruments ", " "and")
  "number" (len .instruments)

)

We’ve also created an instruments maps with various informations that we could use.
Under .list we have the raw list from our content file.
Under string_rep we have a string representation of the list built with the now familiar delimit.
And finally, under .numbers the numbers of instruments. len is a useful function evaluating the length of a slice!

Now if our project needs to sort out gents by the number of instruments they play, it would be just as easy as:

{{ $gents := sort $new_gents "instruments.number" }}

Transform with a partial

That’s really good, but we should really isolate our transforming operations in a returning partial so the above is cleaner:

{{ $new_gents := slice }}
{{ range $gents }}
  {{ $new_gents = $new_gents | append (partial "gent_transformer" .)
{{ end }}

And from that partial we’d return a simple map of our new transformed gent.

{{/* /layouts/partials/transform_gent.html */}}
{{ return dict
    "fullname" .Title
    "firstname" (index 0 (split .Title " "))
    "lastname" (index 1 (split .Title " "))
  [etc...]
}}

Inside our transformer

Nice. Now let’s focus on the content of partials/transform_gent.html. It’s really unsafe of us to assume all those keys are filled.

What if there is no instruments? Then we should probably do not use delimit, and simply return an empty slice!

What if there is no “lastname”? Then our index will definitely fail and break our build as a result.

We cannot simply declare our dict in one shot anymore. We have to increment the additions of key/value pairs based on certain conditions.

Adding to a map? Awesome, we’ve covered it in the last article!

Well… We’ve covered one way to do it with merge. But there is one serious problem with the merge technique: it’s pretty slow. And understandably as for every addition, it invokes two functions, merge and the creation of a new map with dict.

If you only have a few hundred gents, it should be okay, but if you need to transform thousands of them, you’ll end up bumping your build time considerably.
The reasonable approach is Scratch. Scratch is seldom used these days but it remains the best solution to modify maps!

We’ll limit ourselves to the

.SetInMap scratch method. It takes three parameters, the map to modify, the concerned key and the value. If the key exists, it overwrites its value, if it does not exist, it creates it with the given value.

Let’s start… from .Scratch!

{{/* /layouts/partials/transform_gent.html */}}
{{ $s := newScratch }}
{{ $s.Set "gent" dict }}

{{ return $s.Get "gent" }}

  1. First, we store our scratch instance in a $s variable (short for… Scratch!). All its methods and data will be stored in there.
  2. Then we store an empty map in our scratch called "gent"
  3. Right after that we’ll proceed to our various conditions and data manipulations.
  4. At the end, we return the "gent" map stored in the $s Scratch.

Ok let’s start safely transforming our gents and improve our code while we’re at it!

Name

First let’s improve that firstname/lastname thing. Currently it will only work with the most “two words” names like John Lennon but what if a zealous editor entered title: John Winston Lennon. Now our little concoction would use the wrong substring as lastname. Another problem could occur if the editor enters title: Ringo. This time we have a broken build as index . 1 does not exist!

First we need to make sure we have at least 2 strings seperated by a whitespace. We’ll use len to retrieve the number of strings contained in the slice returned by split.

{{ with split .Title " " }}
  {{ if gt (len .) 1 }}
    🎉
  {{ end }}
{{ end }}

Having more than one does not mean we have two, there could be 3 or 5 words in there. Let’s use first and last on the resulting slice! This way we’ll be sure to only get the first and last strings. Because we’re always using with, it’s safe to use index.

{{ with split .Title " " }}
  {{ if gt (len .) 1 }}
    {{ with first 1 . }}
      {{ $s.SetInMap "gent" "firstname" (index . 0) }}
    {{ end }}
    {{ with last 1 . }}
      {{ $s.SetInMap "gent" "lastname" (index . 0) }}
    {{ end }}
  {{ end }}
{{ end }}

Finally we’ll decide what to do if we only have one word in there. I guess it could be the firstname…

{{ with split .Title " " }}
  {{ if gt (len .) 1 }}
  [...]
  {{ else }}
    {{ $s.SetInMap "gent" "firstname" (index . 0) }}
  {{ end }}
{{ end }}

If it’s your first time seeing gt or with, you should probably hit ! It’s a great refresh or introduction on everything Go Templates!

First time seeing first or last? You’ve missed the second part: !

City

Now we need to improve that city assumption. Yes for now we only have gents from Liverpool! But our gents project is bound to scale to more gents and bands!

For now we could simply default to Liverpool but still check for a city Front Matter value.

This is tempting:

{{ $city := "Liverpool" }}
{{ with .Params.city }}
  {{ $city = . }}
{{ end }}
{{ $s.SetInMap "gent" "city" $city }}

There’s much more simpler though with the default function! It takes two parameters, first one is the default value to be used, second one is the input whose value will be tested before using the default.

{{ $s.SetInMap "gent" "city" (default "Liverpool" .Params.city) }}

With the above, if .Params.city is missing from the Front Matter or equals to false or is an emtpy string, our transformed gent’s city will read Liverpool, otherwise, it’ll be whatever the editor wanted it to be!

Birthdate and Age

Nothing new for the birthdate except we condition its addition on the existence of a .Date.

{{ with .Date }}
  {{ $s.SetInMap "gent" "birthdate" . }}
{{ end }}

Easy!

For the age, we’ll perform one easy calculation with the sub function discussed earlier in the series and the now function. The now function simply returns the current time in the form of a Go Date object with among many a .Day, .Month and .Year methods.

{{ with sub now.Year .Date.Year }}
  {{ $s.SetInMap "gent" "age" $age }}
{{ end }}

Now some songs?

Wouldn’t it be nice to list all the songs attributed to those creative gents even though we don’t have that information in our content files?

Now that we are very confortable with handling data, we can try get it from the cloud using .GetRemote!

I set up yet another Beatles API at https://ya-beatles-api.netlify.app/songs. This endpoint will return a JSON array of songs formatted like so:

[
  {
    "name": "A Day in the Life",
    "songwriters": [
      "John Lennon",
      "Paul McCartney"
    ],
    "year": 1967
  },
  {etc...}
]

We want to fetch this data, and list the song names attributed to each of our gents. This should be fun and a nice recap of what we’ve been learning over the course of this series.

{{ with resources.GetRemote "https://ya-beatles-api.netlify.app/songs" }}
  {{ with .Content | unmarshal }}
    {{ $songs := slice }}
    {{ with where . "songwriters" "intersect" (slice $.Title) }}
      {{ range . }}
        {{ $songs = $songs | append .name }}
      {{ end }}
    {{ end }}
    {{ with $songs }}
      {{ $s.SetInMap "gent" "songs" . }}
    {{ end }}
  {{ end }}
{{ end }}
  1. We use resources.GetRemote to fetch the API endpoint.
  2. We turn its content into Hugo data with unmarshal.
  3. We create an emtpy slice for our songs.
  4. Using with, where and intersect we filter all the songs form the API to only keep the ones whose .songwriters includes the name of our gent.
  5. Using range, we loop on our filtered songs and append our $songs array with the .name of the song at cursor.
  6. Using with again, we make sure the above did populate our $songs array and if so, store its value in our local scratch’s “songs” key.
  7. Done!

One more thing to “apply”!

Remember at the beginning of this article when we applied the transformations to our gents with a range:

{{ $new_gents := slice }}
{{ range $gents }}
  {{ $new_gents = $new_gents | append (partial "gent_transformer" .)
{{ end }}

It is not ideal! There is an underated Hugo function called apply we can use instead.
It takes as first argument a slice and as second the “function” to apply. All subsequent arguments are passed to the applied function.

For example we could do:

{{ $gents := "John" "Paul" "George" "Ringo" }}
{{ $gents = apply $gents "printf" "I love %s" "." }}

And our $gents array would now hold the following strings:

[I love John, I love Paul, I love George, I love Ringo]

Now, using apply, we can apply our transformations without a range.

{{ $new_gents := apply $gents "partial" "transform_gent" "." }}

Finally outputing our data!

Now that we’re done formatting or data we can keep the Hugo logic in our template to the minimum. And after reading this series on Hugo and Data, you should be perfectly capable of understanding the following without any help:

<h2>Gents from Britain</h2>
{{ $gents_pages := where site.RegularPages "Type" "gent" }}
{{ $gents := apply $gents_pages "partial" "transform_gent" "." }}
{{ range $gents }}
  <details>
    <summary>{{ .fullname }}</summary>
    <dl>
      {{ range $key, $value := . }}
      <dt>{{ $key }}</dt>
      <dd>
        {{ if reflect.IsSlice . }}
        <ul>
          {{ range . }}
          <li>{{ . }}</li>
          {{ end }}
        </ul>
      {{ else }}
        {{ . }}
      {{ end }}
      </dd>
      {{ end }}
    </dl>
  </details>
{{ end }}

Conclusion

I hope this has been a fun and detailed way to cover everything there is to know about Hugo and Data to start or complement existing data-heavy Hugo projects.

The final code resulting in our series is available to look at at https://github.com/regisphilibert/gents-from-britain
The code for the Beatles API (built with Hugo) is available here: https://github.com/regisphilibert/ya-beatles-api

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