Seven Web Frameworks in Seven Weeks - Version Conflicts

Title: Name of book: Seven Web Frameworks in Seven Weeks (page 5+)
Example: DataMapper has been discontinued, conflict over json versions
book-seven-web-frameworks-in-seven-weeks web-development ruby sinatra #datamapper

There are several issues I want to raise here briefly.

One is that it seems DataMapper has been discontinued. (Could that be contributing to the difficulty I’ve had getting the code to work?)

Another is that I get an error like the following both with testing the Sinatra code and running the crud application:
“rspec app_test.rb
An error occurred while loading ./app_test.rb.
Failure/Error: raise Gem::ConflictError.new self, conflicts
Gem::ConflictError:
Unable to activate dm-serializer-1.2.2, because json-2.5.1 conflicts with json (~> 1.6)”

I’ve messed around a lot with installing different versions of json, or trying to enable temporarily removing the default (2.5.1) so there won’t be a json conflict - but if there’s a magic trick to it, I don’t know what it is.

When I try to run the crud application I get the following lines (and more, but this is the gist of it):

“ruby app.rb
/usr/lib/ruby/vendor_ruby/rubygems/specification.rb:2236:in raise_if_conflicts': Unable to activate dm-serializer-1.2.2, because json-2.5.1 conflicts with json (~> 1.6) (Gem::ConflictError) from /usr/lib/ruby/vendor_ruby/rubygems/specification.rb:1367:in activate’”

I’ve looked at a number of pieces on Stack Overflow and similar resources and am having trouble finding anything that helps - though I’ve tried a variety of tips and suggestions.

Another issue I ran into was that initially I got errors saying that the file sqlite3.h was not found; after a good deal of searching my files, github, stackoverflow, etc., I discovered that if I installed libsqlite3-dev this particular problem went away. This doesn’t appear to have been installed merely through following the steps in the book (presumably the .h file was present or not needed at publishing time).

Any further suggestions would be very welcome. I am trying to learn as much as I can. I’m excited about this book and to be learning about it.