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In Grails / GORM, what is the difference between static mapping = {xyz lazy: false} & static fetchMode = [xyz: 'eager'] ?

Example:

class Book {
    static belongsTo = [author: Author]
    static mapping   = {author  lazy: false}
    static fetchMode = [author: 'eager']
                I think this might answer your question:     [stackoverflow.com/questions/654704/…        [1]: stackoverflow.com/questions/654704/…
– David Chavez
                May 6, 2015 at 13:53
                Thanks for the link, but the answer seems to describe the difference between lazy: true & lazy: false.  The answer doesn't mention fetchMode = [...]
– XDR
                May 7, 2015 at 4:35
                It would be nice it someone could answer this question as I didn't find any explanation on the grails documentation neither on the internet.
– Merlin
                Oct 19, 2015 at 9:17
lazy:false will get the associated domain object by querying again to database using Select Query, but fetchMode 'eager' which is deprecated now(use fetch:'join') will try to join the associated tables(using outer join) and fetch the associated objects in single query.
lazy:false will have one more query to the database to fetch the associated domain object and hence would be having more interactions with the database whereas fetch:'join' will have less interaction to fetch the same data.
FetchMode Join overrides the lazy property. It will simple ignore the lazy:false.
Should you be interested in a detailed explanation about Fetchmodes, take a look http://www.solidsyntax.be/2013/10/17/fetching-collections-hibernate/. The article describes the Hibernate fetchmodes and the output which they produce.
Hope this helps.
        
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