Adding and removing from lists

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Example Events
Let's say we're developing support meetings, where participants can be invited and later accept or reject the invitation. In this scenario a reasonable Projection would be to view all participants that currently have accepted their invitation.
An example of a events for two participants that accepted their invitations could look like this:
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{  
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     "aggregateId":"723ecfce-14e9-4889-98d5-a3d0ad54912f",
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     "events":[  
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        {  
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           "eventId":"127b80b5-4a05-4774-b870-1c9a2e2a27a3",
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           "eventType":"InvitationAcceptedEvent",
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           "data":{  
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              "userId": "673b3926-3667-4daf-9db7-03c08adc5ffd"
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           }
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        }
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     ]
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  }
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{  
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     "aggregateId":"723ecfce-14e9-4889-98d5-a3d0ad54912f",
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     "events":[  
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        {  
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           "eventId":"1a20dfac-ae44-49ac-9491-bcb6d418c4d0",
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           "eventType":"InvitationAcceptedEvent",
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           "data":{  
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              "userId": "fa0d1d7c-9c64-4d65-b000-e1ace3a67bb2"
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           }
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        }
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     ]
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  }
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Later, one of the participants rejects the invitation:
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  {  
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     "aggregateId":"723ecfce-14e9-4889-98d5-a3d0ad54912f",
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     "events":[  
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        {  
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           "eventId":"127b80b5-4a05-4774-b870-1c9a2e2a27a3",
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           "eventType":"InvitationRejectedEvent",
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           "data":{  
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              "userId": "673b3926-3667-4daf-9db7-03c08adc5ffd"
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           }
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        }
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     ]
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  }
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Projection definition
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{
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  "aggregated": false,
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  "projectionName": "meetings",
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  "feedName": "meeting",
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  "handlers": [
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    {
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      "eventType": "InvitationAcceptedEvent",
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      "functions": [
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        {
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          "function": "append",
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          "targetSelector": "$.projection.participants"
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        }
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      ]
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    },
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    {
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      "eventType": "InvitationRejectedEvent",
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      "functions": [
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        {
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          "function": "remove",
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          "targetSelector": "$.projection.participants[?]",
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          "targetFilter": "[?(@.userId == $.event.userId)]"
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        }
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      ]
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    }
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  ]
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}
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Results
/projections/single/meetings/723ecfce-14e9-4889-98d5-a3d0ad54912f
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{
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  "projectionId" : "723ecfce-14e9-4889-98d5-a3d0ad54912f",       
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  "createdAt" : 1523518145532,                                   
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  "updatedAt" : 1523518146253,                                   
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  "data": {
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    "participants": [{
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      "userId": "fa0d1d7c-9c64-4d65-b000-e1ace3a67bb2"
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    }]
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  }
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}
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Aggregating data to a list
API Reference
Last modified 2yr ago
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Contents
Example Events
Projection definition
Results