How can I group entries based on a dateTime field?

Question

I am getting all entries from a section like this:

// get the parent entry.
{% set author = craft.entries.section('mySection').authorId(currentUser.id).first() %}

// get the child entries from the parent.
{% set entires = craft.entries({
    'section': 'mySection',
    'level': 2,
    'descendantOf': author.id
 })%}

// myDate is a dateTime filed (yyyy-mm-dd)

{% for year, entry in entries|group('myDate') %}
            <h4>{{ year|date('Y') }}</h4>
            <ul>
                {% for entry in entries %}
                    {% if attempt.myDate|date('Y') == year|date('Y') %}
                        <li>{{ entry.myDate }} > {{ entry.type.name }} - {{ entry.foo }}</li>
                    {% endif %}
                {% endfor %}
            </ul>
        {% endfor %}

What I have is close, however, it's outputting multiple years. For example,

2016
  [entry]

2016
  [entry]

2015
  [entry]

 ...

I'm trying to group all of the child entries by year. For example, if I have 3 entires with the same year (2016) I am trying to show:

2016
  [entry]
  [entry]
  [entry]

2015
  [entry]

2014
  [entry]
  [entry]

...

Thank you for any suggestions!

Answer 1

You're currently grouping on the myDate field, which essentially creates a new array index for each date (e.g. 2016-01-01) which is why you're getting your current output.

To group by year, this should do the trick:

{% for year, entries in entries|group('myDate.year') %}
    <h4>{{ year }}</h4>
    <ul>
        {% for entry in entries %}
            ...
        {% endfor %}
    </ul>
{% endfor %}

Answer 2

I think that the official Craft CMS documentation provides an answer to your question really clearly: https://craftcms.com/support/entry-archive

For example,

{% set childEntires = craft.entries({
    'section': 'mySection',
    'level': 2,
    'descendantOf': author.id
})%}

{% for date, entries in childEntires | group("myDate|date('Y')") %}
    <h2>{{ date }}</h2>
    <ul>
        {% for entry in entries %}
            <li>{{ entry.myDate | Y }} > {{ entry.type.name }} - {{ entry.foo }}</li>
        {% endfor %}
    </ul>
{% endfor %}