how to map objects into arrays in a query

Question

Let's suppose I have a collection named "repos" containing objects like this one:

{
  name: 'myrepo',
  actions: [
     { timestamp: '2016-04-12T14:43:20Z', change: 'add' },
     { timestamp: '2016-04-12T14:45:10Z', change: 'remove' },
     { timestamp: '2016-04-12T15:03:03Z', change: 'add' },
     ... and so on ....
  ]
}

Now I want a query to convert each of these objects into something like this:

{
   name: 'myrepo',
   timestamps: ['2016-04-12T14:43:20Z', '2016-04-12T14:45:10Z', '2016-04-12T15:03:03Z'],
   changes: ['add', 'remove', 'add']
}

I have thought of something like the following:

FOR r in repos
LET changes= (FOR a IN r.actions RETURN a.change )
LET timestamps = (FOR a IN r.actions RETURN a.timestamp)
RETURN {
    name: r.name,
    changes: changes,
    timestamps: timestamps
    }

but I am afraid that the double FOR may be not very efficient.

Any suggestion?

CodeManX · Accepted Answer · 2016-04-12 23:58:22Z

2

You can express your query in a more compact form using the array expansion operator:

FOR r IN repos RETURN {
  name: r.name,
  changes: r.actions[*].change,
  timestamps: r.actions[*].timestamp
}

You may also use UNIQUE to only get every change type once per record in your result (if the arrays changes and timestamps don't have to lineup):

FOR r IN repos RETURN {
    name: r.name,
    changes: UNIQUE(r.actions[*].change),
    timestamps: r.actions[*].timestamp
}

In general, your query only adds the overhead of two sub-queries, and shouldn't be that much slower than the above. However, it's more compact and therefore a little bit better to read, isn't it?

edited Apr 12, 2016 at 23:58

CodeManX

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answered Apr 12, 2016 at 16:38

dothebart

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