merge-lists.md

description
Blend two input lists into one using a shared identifier.

Merge lists

Use case

You're faced with two separate lists of data that need to be combined based on a common attribute. You require a merging method that mimics SQL JOINs.

Overview

The Merge Lists transform equips you with the functionality to effectively merge two lists into one. By aligning items based on a shared key attribute and allowing for three types of merging strategies (inner, left, and outer), it enhances your data analysis and manipulation capabilities.

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Parameters

Here are the parameters you have available to you within the action, and their descriptions:

Parameter	Description	Required
Merge Method	Choose the strategy to merge the lists. Options are inner, left, or outer join.	true
First List	Enter the first list for the merging process.	true
First List's Key	Identify the key attribute for matching items in the first list.	true
Second List	Enter the second list to be merged with the first one.	true
Second List's Key	Identify the key attribute for matching items in the second list.	true

Usage

Input lists

For this transform, we will want to provide the action two lists and their corresponding key's to be used for mapping the comparisons. Let's use the below for our examples:

List 1:

list_1: [
  {"id": 1, "name": "John"},
  {"id": 2, "name": "Mary"}
]

List 2:

list_2: [
  {"id": 1, "age": 30},
  {"id": 3, "age": 35}
]

Merge methods

This transform offers three different methods for merging your lists: Inner, Left, and Outer joins. These three methods mimic the functionality of SQL JOINs and each has its own use cases:

1. Inner join: useful when you're dealing with two datasets and only want to focus on data that is common between them.
2. Left join: useful when the first list is your primary dataset and you wish to append any additional, relevant data from the second list to it.
3. Outer join: useful when you aim for a comprehensive view, combining all data from both lists, and filling in gaps where possible.

Here are some examples of these methods in action to help you better understand their operation:

Inner join

The Inner merge method is used when you only want to retain the entries that are present in both lists. The intersection is based on the values of the specified keys.

Action Parameters:

join_method: inner
list_1: list_1
list1_key: id
list_2: list_2
list2_key: id

Jinja2 Equivalent:

jinjaCopy code
{% raw %}
{% set result = [] %}
{% for item1 in list_1 %}
  {% for item2 in list_2 %}
    {% if item1[list1_key] == item2[list2_key] %}
      {% do result.append(item1 | combine(item2)) %}
    {% endif %}
  {% endfor %}
{% endfor %}
{% endraw %}

Left join

The Left merge method is employed when you want to keep all the entries from the first list and incorporate matching entries from the second list.

Parameters:

join_method: left
list_1: List 1
list1_key: id
list_2: List 2
list2_key: id

Jinja2 Equivalent:

{% raw %}
{% for item1 in list_1 %}
  {% for item2 in list_2 %}
    {% if item1[list1_key] == item2[list2_key] %}
      {% do item1.update(item2) %}
    {% endif %}
  {% endfor %}
{% endfor %}
{% endraw %}


{{ list_1 }}

Outer join

The Outer merge method is used when you want to keep all entries from both lists, regardless of whether they have a match.

Parameters:

join_method: outer
list_1: List 1
list1_key: id
list_2: List 2
list2_key: id

Jinja2 Equivalent:

{% raw %}
{% for item1 in list_1 %}
  {% for item2 in list_2 %}
    {% if item1[list1_key] == item2[list2_key] %}
      {% do item1.update(item2) %}
    {% endif %}
  {% endfor %}
{% endfor %}
{% endraw %}
{{ list_1 + [item2 for item2 in list_2 if all(item2[list2_key] != item1[list1_key] for item1 in list_1)] }}

Results output

The outputs of the three different merge method examples above can be seen as follows:

Inner join: only John's object is returned in the output, as his id is present in both lists.

results: [
  {"id": 1, "name": "John", "age": 30}
]

Left join: all objects from list_1 are returned, with matching objects from list_2 added on. This is why John's object includes the age from list_2, but Mary's object remains the same, as there was no matching id in list_2.

results: [
  {"id": 1, "name": "John", "age": 30},
  {"id": 2, "name": "Mary"}
]

Outer join: all objects from both lists are returned, with matching objects merged together. Therefore, we get John's object with the age from list_2, Mary's object from list_1, and the object with id 3 from list_2 which didn't have a matching id in list_1.

results: [
  {"id": 1, "name": "John", "age": 30},
  {"id": 2, "name": "Mary"},
  {"id": 3, "age": 35}
]

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Now that you're equipped with the knowledge of Merge Lists transform, you're prepared to blend data from two different lists into a coherent whole. Keep in mind your merging strategy (inner, left, or outer) as it directly impacts your resulting list.