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Adding backlinks

This example shows how to handle a schema that makes use of a backlink. We’ll use a linked-list structure to represent a sequence of events.

We’ll start with this schema:

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type Event {
  required property name -> str;
  link prev -> Event;

  # ... more properties and links
}

We specify a prev link because that will make adding a new Event at the end of the chain easier, since we’ll be able to specify the payload and the chain the Event should be appended to in a single insert. Once we’ve updated the schema file we proceed with our first migration:

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$ 
edgedb migration create
did you create object type 'default::Event'? [y,n,l,c,b,s,q,?]
> y
Created ./dbschema/migrations/00001.edgeql, id:
m1v3ahcx5f43y6mlsdmlz2agnf6msbc7rt3zstiqmezaqx4ev2qovq
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$ 
edgedb migrate
Applied m1v3ahcx5f43y6mlsdmlz2agnf6msbc7rt3zstiqmezaqx4ev2qovq
(00001.edgeql)

We now have a way of chaining events together. We might create a few events like these:

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db> 
... 
... 
... 
select Event {
    name,
    prev: { name },
};
{
  default::Event {name: 'setup', prev: {}},
  default::Event {name: 'work', prev: default::Event {name: 'setup'}},
  default::Event {name: 'cleanup', prev: default::Event {name: 'work'}},
}

It seems like having a next link would be useful, too. So we can define it as a computed link by using backlink notation:

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type Event {
  required property name -> str;

  link prev -> Event;
  link next := .<prev[is Event];
}

The migration is straightforward enough:

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$ 
edgedb migration create
did you create link 'next' of object type 'default::Event'?
[y,n,l,c,b,s,q,?]
> y
Created ./dbschema/migrations/00002.edgeql, id:
m1qpukyvw2m4lmomoseni7vdmevk4wzgsbviojacyrqgiyqjp5sdsa
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$ 
edgedb migrate
Applied m1qpukyvw2m4lmomoseni7vdmevk4wzgsbviojacyrqgiyqjp5sdsa
(00002.edgeql)

Trying out the new link on our existing data gives us:

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db> 
... 
... 
... 
... 
select Event {
    name,
    prev_name := .prev.name,
    next_name := .next.name,
};
{
  default::Event {
    name: 'setup',
    prev_name: {},
    next_name: {'work'},
  },
  default::Event {
    name: 'work',
    prev_name: 'setup',
    next_name: {'cleanup'},
  },
  default::Event {
    name: 'cleanup',
    prev_name: 'work',
    next_name: {},
  },
}

That’s not quite right. The value of next_name appears to be a set rather than a singleton. This is because the link prev is many-to-one and so next is one-to-many, making it a multi link. Let’s fix that by making the link prev a one-to-one, after all we’re interested in building event chains, not trees.

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type Event {
  required property name -> str;

  link prev -> Event {
    constraint exclusive;
  };
  link next := .<prev[is Event];
}

Since the next link is computed, the migration should not need any additional user input even though we’re reducing the link’s cardinality:

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$ 
edgedb migration create
did you create constraint 'std::exclusive' of link 'prev'?
[y,n,l,c,b,s,q,?]
> y
Created ./dbschema/migrations/00003.edgeql, id:
m17or2bfywuckdqeornjmjh7c2voxgatspcewyefcd4p2vbdepimoa
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$ 
edgedb migrate
Applied m17or2bfywuckdqeornjmjh7c2voxgatspcewyefcd4p2vbdepimoa
(00003.edgeql)

The new next computed link is now inferred as a single link and so the query results for next_name and prev_name are symmetrical:

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db> 
... 
... 
... 
... 
select Event {
    name,
    prev_name := .prev.name,
    next_name := .next.name,
};
{
  default::Event {name: 'setup', prev_name: {}, next_name: 'work'},
  default::Event {name: 'work', prev_name: 'setup', next_name: 'cleanup'},
  default::Event {name: 'cleanup', prev_name: 'work', next_name: {}},
}
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