twigfiddle.com - Fiddle j048k5 v1 - Created on September 23, 2020 10:41

File

{{ forecasts }}

{% set data = {} %}
{% for forecast in forecasts %}
  {% for weather in forecast.weathers %}
    {% if data[weather.dateTime] is defined %}
      {% set dataDay = data[weather.dateTime] %}
    {% else %}
      {% set dataDay = [] %}
    {% endif %}
    {% set dataDay = dataDay|merge([{'cityId': forecast.city.id, 'weather': weather.weather, 'temperatureMin': weather.temperatureMin, 'temperatureMax': weather.temperatureMax}]) %}
    {% set data = data|merge({(weather.dateTime):dataDay}) %}
  {% endfor %}

{% endfor %}

{{ data|json_encode|raw }}

Format

{"forecasts": [
  {
     "city": {
         "id": 1,
         "name": "yo",
         "latitude": "yo",
         "longitude": "yo",
         "zipcode": "yo",
         "country": "yo"
     },
     "weathers": [
        {
            "weather": "clear-sky",
            "dateTime": "2020-09-21",
            "temperatureMin": 0,
            "temperatureMax": 10
        },
        {
            "weather": "clear-sky",
            "dateTime": "2020-09-22",
            "temperatureMin": 0,
            "temperatureMax": 10
        }
     ]
  },
  {
     "city": {
         "id": 2,
         "name": "yo",
         "latitude": "yo",
         "longitude": "yo",
         "zipcode": "yo",
         "country": "yo"
     },
     "weathers": [
        {
            "weather": "clear-sky",
            "dateTime": "2020-09-21",
            "temperatureMin": 50,
            "temperatureMax": 550
        },
        {
            "weather": "clear-sky",
            "dateTime": "2020-09-22",
            "temperatureMin": 60,
            "temperatureMax": 660
        }
     ]
  }
    ]
}