PG undefinedtable error relation users does not exist

At first, you shall detach all connections out of database. By default you use the development environment. Then try to reset database with the following:

rake db:reset

The rake db:reset task will drop the database and set it up again. This is functionally equivalent to rake db:drop db:setup.

This is not the same as running all the migrations. It will only use the contents of the current schema.rb file. If a migration can't be rolled back, rake db:reset may not help you. To find out more about dumping the schema see Schema Dumping and You section. Rails Docs

If the trick doesn't help, drop the database, then re-create it again, migrate data, and if you have seeds, sow the database:

rake db:drop db:create db:migrate db:seed

or in short way (since 3.2):

rake db:migrate:reset db:seed

Since db:migrate:reset implies drop, create and migrate the db. Because the default environment for rake is development, in case if you see the exception in spec tests, you should re-create db for the test environment as follows:

RAILS_ENV=test rake db:drop db:create db:migrate

or with just loading the migrated scheme:

RAILS_ENV=test rake db:drop db:create db:schema:load

In most cases the test database is being sowed during the test procedures, so db:seed task action isn't required to be passed. Otherwise, you shall to prepare the database (this is deprecated in Rails 4):

rake db:test:prepare

and then (if it is actually required):

RAILS_ENV=test rake db:seed

On newer versions of Rails the error ActiveRecord::NoEnvironmentInSchemaError may be risen, so just prepend the tasks with a database environment set task: db:environment:set:

RAILS_ENV=test rake db:environment:set db:drop db:create db:migrate

I encountered this error, and upon my research, found out that one of the reasons for PG undefinedtable error relation users does not exist error is:

This error is a migration error. You may have created new model with some database attributes. After creating model you have to migrate attributes to your rails app schema.

If you are using local machine, for development, you can use command

rake db:migrate

If you're using heroku

heroku run rake db:migrate

Your test database is not ready for rspec.

Prepare your test database for rspec to fix this error

RAILS_ENV=test rake test:prepare

It will drop, create and add migrations to your test database

In case rake task is aborted with message like 'PG::Error: ERROR: database "[your_db_test]" is being accessed by other users' execute this one

RAILS_ENV=test rake db:migrate