···

       
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100
       
 
       
However, this isn't quite enough yet to store the relations from GraphQL results. The normalized

     

       
101
       
101
       
 
       
cache must also generate primary keys for each entity so that it can store them in table-like data

     

       
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102
       
 
       
structures. This is for instance why [Relay

     

       
103
       
103
       
-
       
enforces](https://relay.dev/docs/en/graphql-server-specification.html#object-identification) that

     

       
103
       
103
       
+
       
enforces](https://relay.dev/docs/guides/graphql-server-specification/#object-identification) that

     

       
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104
       
 
       
each entity must have an `id` field. This allows it to assume that there's an obvious primary key

     

       
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for each entity it may query. Instead, `urql`'s Graphcache and Apollo assume that there _may_ be an

     

       
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`id` or `_id` field in a given selection set. If Graphcache can't find these two fields it'll issue