PragProg Customers

Web Development with Clojure, Third Edition: (create-migration "guestbook") not working (page 9)

simonpeter

simonpeter

Web Development with Clojure, Third Edition: (create-migration "guestbook") not working (page 9)

When I try the command to create a pair of migration files I get an error.

user=> (create-migration "guestbook")
Execution error (IllegalArgumentException) at user/create-migration (user.clj:61).
find not supported on type: mount.core.DerefableState

user=>

Linux Ubuntu 20.04 LTS, OpenJDK 14.0.2

4 comments
#errata/book-web-development-with-clojure-third-edition
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Marked As Solved

bishax

bishax

This is probably because you haven’t mounted the DB by starting the app.

(in-ns 'guestbook.user)
(start)
(create-migration "foo")
1
Post #2

Also Liked

simonpeter

simonpeter

Thank you, I tried that and it worked.

Hopefully that can be added to the instructions in the book so that people like me who are learning Clojure as they are learning how to make web apps using it can be less confused.

2
Post #3
bmads_51

bmads_51

Dmitri Sotnikov @dmitri and Scot Brown @svmbrown

Working through this now and encountered same issue, so thanks for this thread!

In addition, I found on p. 9

A common beginner error that you may encounter is forgetting to start the
database connection. If you do get this error, don’t worry—just run (start) and try again. In fact, any error that mentions mount.core.DerefableState is probably due to forgetting to run (start).

But I agree. There should be an additional sentence telling the reader to not only start the REPL (which it does) but to also ‘(start)’ especially since we are a long way from p. 9!

1
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