Mongo Query: GraphLookup or Aggregate

Question

Looking for some direction on how to construct a mongo query for a graph network visualization.

I have a data model that looks like this:

stories[
{
  story: "story 1",
  theme_ids: [1,2,3],
  author_ids: [a,b,c]
},
{
  story: "story 2",
  theme_ids: [4],
  author_ids: [a]
}
...
]

I would like the result of the query to grab all of the pairs of themes and authors like so:

[
{from: 1, to: a},
{from: 1, to: b},
{from: 1, to: c},
{from: 2, to: a},
{from: 2, to: b},
{from: 2, to: c},
{from: 3, to: a},
{from: 3, to: b},
{from: 3, to: c},
{from: 4, to: a}
]

Thank you! MongoDB v3.4

Answer 1

You can use double $unwind to get all pairs:

db.col.aggregate([
    {
        $project: {
            from: "$theme_ids",
            to: "$author_ids"
        }
    },
    {
        $unwind: "$from"
    },
    {
        $unwind: "$to"
    }
])

If you need to remove the duplicates you can add $group with $addToSet :

{
    $group: {
        _id: null,
        uniquePairs: { $addToSet: "$$ROOT" }
    }
}