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Editorial Team
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Asked: 2026-05-20T11:28:04+00:00

I was curious about how the MongoDB plugin for Grails would handle relationships. In

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I was curious about how the MongoDB plugin for Grails would handle relationships. In order to test this I made a very simple application with two domain classes:

Authors have two fields: String firstName and String lastName

Books have two fields: String title and Author author

After setting up MongoDB and Grails I made some Authors and Books and took a peek using the MongoDB interactive shell. What I found is that the relationships were being handled the same way they would be handled in a relational database: references to other objects’ id fields.

So now for the questions:

  1. In order for GORM to pull this off, does it need a separate connection to retrieve each document?
  2. If yes, wouldn’t this be better off in a relational database such as PostgreSQL or MySQL?
  3. If the answer to the above two questions is indeed ‘yes,’ then is there a better way to manage relationships in a document database such as MongoDB? I realize MongoDB isn’t supposed to be relational, but there are some things that I don’t see how to get around relationships without duplicating data (thereby making update nightmares).

Edit: I also just noticed that grails is not sorting properly on the ‘id’ property of my authors. Does this have to do with using MongoDB? In the shell I can see that the _id property of all the documents made by Grails is of the datatype NumberLong.

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1 Answer

  1. Editorial Team

    I realize MongoDB isn’t supposed to be relational, but there are some things that I don’t see how to get around relationships without duplicating data

    Then don’t sweat it. MongoDB is not anti-relational, it’s document-oriented.

    In this case, Books and Authors are two top-level objects. It’s not reasonable to nest either of them, they are both core entities in their own right.

    In the case of each Book having only one Author (N:1), it’s completely reasonable for the Book to contain a “Reference To” the Author. Sure you’ll have to do two queries. But is that terribly different from doing a join query? The join query still has to do two index look-ups and two data lookups. So you’re not really costing yourself anything here.

    In the case of each Book supporting multiple Authors (M:N), then you have several options based on your needs.

    I don’t like to think of MongoDB as “not relational”, I think it’s cleaner to think of MongoDB as query-optimized.

    I also just noticed that grails is not sorting properly on the ‘id’ property of my authors…

    I would check directly with the Grails author. Sounds like they may be storing “strings” instead of actual ObjectIds (or MongoIDs). While not critical this may be a bug.

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