How can I install bundler for a specific ruby version?

I'm creating a rails app and when I try to run rake db:migrate I get this error:

LoadError: cannot load such file -- bundler/setup

I'm using rbenv and ruby version 2.5.1 in a mac with OS Mojave 10.14.4

I've tried installing bundle with gem install bundleAnd it installs it successfully, but then when I try to run bundle install, I get the message:

rbenv: bundle: command not found
The `bundle' command exists in these Ruby versions:
2.6.2

I've tried making sure I'm currently using ruby 2.5.1 checking with ruby -v and also creating an environment variable to define what version to use with:

 export RBENV_VERSION=2.5.1

Also when I type rbenv versions I get:

system
* 2.5.1
2.6.2

Showing I'm using 2.5.1

I suspect there might be two 2.5.1 versions of ruby installed in different places, maybe one with rvm (which I already uninstalled with rmv implode but didn't know I had). Or I might be missing some rbenv configurations.

But nothing seems to work, apparently gem install installs bundle for ruby 2.6.2 only. How can I fix this?

I'm not 100% sure of everything I'm doing, so far I've just followed online tutorials, so please help me understand what's going on and what to do.

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