Merged keys

Configure how the data is merged

As explained in Shared data, the data assigned to every page is the result of merging the data directly assigned to the page (like the front matter) with the data of the parent folders (stored in the _data files and folders).

├── _data.yaml
├── ...
└── documents
    └── _data.json
    └── ...
    └── examples
        └── _data.json
        └── my-page.md

In this example the /documents/examples/my-page.md file contains all data from the page front matter, merged with the data from /documents/examples/_data.json, documents/_data.json and /_data.yaml files, in this order of priority.

Object mode merging

On merging variables, the complete value is overridden. But you may want to merge some values in a different way. For example, let's say we have the following two data files, one in the root and the other in a subfolder. Both files have a site variable with different values:

site:
  title: My humble site
  author: Oscar Otero

All pages in the subfolder (and sub-subfolders) will have the latest version of the variable that has the author subkey but missing the title value, because the whole variable is overridden. You can change this behaviour using the special value mergedKeys. This value indicates to Lume how to merge some keys:

mergedKeys:
  site: object

site:
  title: My humble site
  author: Oscar Otero

In this example, we are indicating to Lume that the variable site must be merged using the object mode. Now, the result of the variable site is an object including the properties of the parent variable and only overrides the properties with the same name. So the result will be something like this:

site:
  title: My humble site
  author: Laura Rubio

The object merge mode is not recursive; it only works with the first-level properties. A recursive option may be added in the future.

The mergedKeys variable is also merged with other mergedKeys variables in subfolders and pages and using always the object mode. This means that you can define this variable in the root _data file of the site and override it in specific subfolders.

array mode

There's another merge mode for arrays. In this mode, the merge result is an array with all values found in all _data levels. For example:

mergedKeys:
  category: array

category:
  - programming
  - deno
  - javascript

The result of this merge is:

category:
  - programming
  - deno
  - javascript
  - typescript

It's an array with all values present in all parent _data contexts. The result includes only unique values: if the same value is repeated in different _data contexts, it's included only once in the result.

stringArray mode

There's a special case in which you want to make sure that all values of the array are strings. Let's see the following example:

mergedKeys:
  category: array

category:
  - errors
  - 404

The result of this merge is:

category:
  - errors
  - 404
  - "404"

As you can see, the value 404 is duplicated, once as a number and again as a string. To prevent this behavior, you may want to convert all values to strings to remove duplicates. Instead of array, use the stringArray mode:

mergedKeys:
  category: stringArray

Now, the result of this merge is:

category:
  - errors
  - "404"

For backward compatibility, Lume assigns the stringArray merge mode to the key tags automatically.