While working on provisioning matcharoo, a Rails 7.1 app, with Kamal, I hit a problem. Bundler wouldn’t let me deploy because of the following error with the pg gem: Can't find the 'libpq-fe.h header.

Here’s the full log output when I ran kamal setup:

% kamal setup
# .......
#16 43.33 Gem::Ext::BuildError: ERROR: Failed to build gem native extension.
#16 43.33
#16 43.33     current directory: /usr/local/bundle/ruby/3.2.0/gems/pg-1.5.4/ext
#16 43.33 /usr/local/bin/ruby extconf.rb
#16 43.33 Calling libpq with GVL unlocked
#16 43.33 checking for pg_config... no
#16 43.33 checking for libpq per pkg-config... no
#16 43.33 Using libpq from
#16 43.33 checking for libpq-fe.h... no
#16 43.33 Can't find the 'libpq-fe.h header
#16 43.33 *****************************************************************************
#16 43.33
#16 43.33 Unable to find PostgreSQL client library.
#16 43.33
#16 43.33 Please install libpq or postgresql client package like so:
#16 43.33   sudo apt install libpq-dev
# .......
# 43.33 An error occurred while installing pg (1.5.4), and Bundler cannot continue.

Rails 7.1 apps are generated with a standard Dockerfile.

All you need to do is add libpq-dev to the list of Ubuntu dependencies needed for building gems and also to the list of packages needed for deployment.

Other distros will have their own packages.

If you add libpq-dev to the list of deps needed for building gems but omit it from the list of packages needed for deployment, your app container won’t start.

It will fail with:

LoadError: libpq.so.5: cannot open shared object file:
No such file or directory - /usr/local/bundle/ruby/3.2.0/gems/pg-1.5.4/lib/pg_ext.so (LoadError)

Therefore, make sure to add libpq-dev to both places:

# Dockerfile

# Install packages needed to build gems
RUN apt-get update -qq && \
    apt-get install --no-install-recommends -y build-essential git libvips pkg-config \

# Install packages needed for deployment
RUN apt-get update -qq && \
    apt-get install --no-install-recommends -y curl libsqlite3-0 libvips \
    libpq-dev && \
    rm -rf /var/lib/apt/lists /var/cache/apt/archives

Then, make sure to kill your currently running DB container with:

kamal accessory remove db

Then re-run kamal setup again and the blocker will go away!

kamal setup

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