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I saw this question up before, but only for rspec. I haven't created test yet because it's too advanced for me but one day soon i will! :P

I get this error when I try to sign-up/login into my app. I used devise to create user and also omniauth2 to sign-in with google .

this is the error

ActiveRecord::StatementInvalid at /users/auth/google_oauth2/callback
PG::UndefinedTable: ERROR:  relation "users" does not exist
LINE 5:              WHERE a.attrelid = '"users"'::regclass
:             SELECT a.attname, format_type(a.atttypid, a.atttypmod),
                     pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
              FROM pg_attribute a LEFT JOIN pg_attrdef d
                ON a.attrelid = d.adrelid AND a.attnum = d.adnum
             WHERE a.attrelid = '"users"'::regclass
               AND a.attnum > 0 AND NOT a.attisdropped
             ORDER BY a.attnum
I tried rake db:migrate, but it already is created: in schema table users exist. Has anyone got this error before?
database.yml
config=/opt/local/lib/postgresql84/bin/pg_config
development:
  adapter: postgresql
  encoding: unicode
  database: tt_intraweb_development
  pool: 5
  username: my_username
  password:
test:
  adapter: postgresql
  encoding: unicode
  database: tt_intraweb_test
  pool: 5
  username: my_username
  password:
production:
  adapter: postgresql
  encoding: unicode
  database: tt_intraweb_production
  pool: 5
  username: my_username
  password:
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
                for me - RAILS_ENV=test rake db:drop db:create db:schema:load works, thanks for answer. Great
– AMIC MING
                Apr 27, 2016 at 18:22
                Thank you Mr. you just save my life. I try to drop, create, migrate, setup but not any of those commands works until I try you command rake db:reset. The magic happens man!
– David
                Jan 29, 2019 at 7:49
                I accidentally dropped my PG tables and got this error when attempting to migrate.  changing/altering migration and schema files have always been an anxiety for me. For me, I deleted a table and for some reason, PG still recognized it in the schema.  I ran rails db:reset just to drop and create everything locally and it realigned everything.
– Kelvin
                Nov 29, 2021 at 18:58
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
                I also faced this when I made a change to my Table name from article to blog_article instead of changing the model class name to BlogArticle I still left the name of the model class as Article
– Promise Preston
                Feb 20, 2021 at 17:26
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
I was getting this error as well when running rspec:
 Failure/Error: it { expect(subject.priority_list).to eq [nil] * 9 }
 ActiveRecord::StatementInvalid:
   PG::UndefinedTable: ERROR:  relation "priorities" does not exist
   LINE 5:              WHERE a.attrelid = '"priorities"'::regclass
It was resolved for me after I ran
rake db:test:prepare
rake db:test:load
I had a similar error. The root of my error was that I had a reference to a Rails model in my factories.rb file. So it caused a load error issue. The fix was to wrap the reference in a block or {} so that it delays running it.
Here was the BROKEN code:
FactoryGirl.define do
  factory :user do
    guid User.new.send(:new_token)
And it was erroring because User was not defined when factories.rb was being loaded. I wrapped the User.new call in a block and it solved the issue:
Fixed code:
FactoryGirl.define do
  factory :user do
    guid { User.new.send(:new_token) }
Note: probably not best practice to need to call your model like this, but it was a solution to DRY up my code.
                Can anyone please explain why it's "probably not best practice to need to call your model"?  I agree with the DRY benefits...what are the drawbacks?  Thanks!
– theUtherSide
                Feb 18, 2016 at 18:36
This is often caused by a bug in ActiveAdmin. Here's how to get around the bug:
If you're using ActiveAdmin, whichever table PG says doesn't exist, comment out the contents of that ActiveAdmin rb file.
For example, for this case PGError: ERROR: relation "users" does not exist, comment out the entire contents of app/admin/users.rb, then uncomment after you've done your migrations.
That issue for me was being caused by Factory Girl rails. I would recommend for those using it to rename the specs/factories folder to specs/temp and attempting
RAILS_ENV=your_environment bundle exec rake db:migrate --trace
If it passes, then you just found what was causing it. A quick dig through the Factory Girl Rails gem github repo helped me identify the issue.
The factories were failing because I was trying to instantiate a Model that didn't exist upon running! Code sample below: 
FactoryGirl.define do
  factory :billing_product, class: 'Billing::Product' do
    name            Faker::Cat.name
    product_type    'fuel'
    active          true
    payment_options [Billing::PaymentOption.new(term: 1, payment_term: 1)]
Encapsulating the Array in a block (adding {}) did the fix for me. Note that payment_options can take more than one payment option in the example...
payment_options {[Billing::PaymentOption.new(term: 1, payment_term: 1)]}
Refer to the Dynamic Attributes part of the Factory Girl Rails docs for more info.
Don't forget to rename your factories folder back!
I was facing the same problem and then I discovered the following solution.
Make sure You have entered all of the following credentials in the database.yml file and they are correct:
development:
 adapter: postgresql
 encoding: unicode
 database: my_database
 host: localhost
 port: 5432
 pool: 5
 username: postgres
 password: xyz
test:
 adapter: postgresql
 encoding: unicode
 database: my_test_database
 host: localhost
 port: 5432
 pool: 5
 username: postgres
 password: xyz 
I had this problem after I deleted the users table. solutions was changing
change_table(:users)
create_table(:users)
after wasting hours I finally figured out that migration is missing ::Migration[5.0].
Erroneous Migration:
class CreateRoles < ActiveRecord # <---- Pay attention
  def change
    create_table :roles do |t|
      t.string :name
      t.integer :code, limit: 2
      t.boolean :is_active, default: true
      t.timestamps
Fixed and Correct Migration
class CreateRoles < ActiveRecord::Migration[5.0]
  def change
    create_table :roles do |t|
      t.string :name
      t.integer :code, limit: 2
      t.boolean :is_active, default: true
      t.timestamps
This could be a bug with rails and might help someone, instead of struggling and wondering.
                yes!!! I don't think it's a bug though it is a common issue for rails 5. I think rails purposely did this Directly inheriting from ActiveRecord.
– Community
                Feb 14, 2019 at 9:51
I was getting a similar error while trying to run tests using rspec.
I followed  Малъ Скрылевъ's steps but still ended up short. The final step I needed to do was load my schema into my test database using: 
RAILS_ENV=test rake db:schema:load
After that the problem went away and I could move on to the next bug. Hopefully that gives you some insight.

(I know this is old, but for future googlers)
Are you using devise? I know specifically omniauthable is a problem, but maybe others as well. It doesn't have to be devise though. Generically the solution is to comment out the offending model, class, whatever, and un-comment any sections the errors ask for.
For me, what was happening is that devise is reading the User model to see what you have as arguments for devise (the class method
i.e. devise :database_authenticatable, :registerable #etc)
But, it will read the whole file and if this isn't a new project it might get tripped up by other class methods relying on other things (in my case it was the friendly_id gem, and then an alias_method
The answer was to comment out the User model except for the devise lines(s) * and rake db:schema:load should run fine.
otherwise I got this error:
ArgumentError: Mapping omniauth_callbacks on a resource that is not omniauthable
Please add devise :omniauthable to the User model
I had this problem and it turned out to be caused by Grape API. I noticed in the stack trace that the routes file was being read during the migration.
In routes.rb the Grape api is mounted
mount API::Base => '/'
And in the API were references to the missing model. So, thanks to this answer I put it in a block that detects whether its being run by the server or during the migration.
unless ( File.basename($0) == "rake" && ARGV.include?("db:migrate") )
    mount API::Base => '/'
And it worked.
I was having the following error and doing a lookup into all my application code for type_zones I was unable to find it. I also looked at the db and it was updated.
Turns out it was a file under fixtures /test/fixtures/type_zones.yml that was causing the trouble.
ERROR["test_should_get_new", UsersControllerTest, 0.47265757399145514]
test_should_get_new#UsersControllerTest (0.47s)
ActiveRecord::StatementInvalid:  ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  relation "type_zones" does not exist
    LINE 1: DELETE FROM "type_zones"
    : DELETE FROM "type_zones"
For anyone who is still having this problem, in my case, it was my factory in FactoryGirl that was triggering this error. 
I was trying to add reference via '.new' or '.create'.
java version "1.7.0_25"
Java(TM) SE Runtime Environment (build 1.7.0_25-b15)
Java HotSpot(TM) 64-Bit Server VM (build 23.25-b01, mixed mode)
=> Solr is already running
rake aborted!
PG::UndefinedTable: ERROR:  relation "discussions" does not exist
LINE 5:              WHERE a.attrelid = '"discussions"'::regclass
:             SELECT a.attname, format_type(a.atttypid, a.atttypmod),
                 pg_get_expr(d.adbin, d.adrelid), a.attnotnull, a.atttypid, a.atttypmod
          FROM pg_attribute a LEFT JOIN pg_attrdef d
            ON a.attrelid = d.adrelid AND a.attnum = d.adnum
         WHERE a.attrelid = '"discussions"'::regclass
           AND a.attnum > 0 AND NOT a.attisdropped
         ORDER BY a.attnum
/Users/matthewcampbell/Sites/code/stack-builders/AchieveX/app/admin/users.rb:25:in block in <top (required)>'
/Users/matthewcampbell/Sites/code/stack-builders/AchieveX/app/admin/users.rb:1:in'
/Users/matthewcampbell/Sites/code/stack-builders/AchieveX/config/routes.rb:3:in block in <top (required)>'
/Users/matthewcampbell/Sites/code/stack-builders/AchieveX/config/routes.rb:1:in'
/Users/matthewcampbell/Sites/code/stack-builders/AchieveX/config/environment.rb:5:in `'
Tasks: TOP => db:migrate => environment
(See full trace by running task with --trace)
I commented out the app/admin/discussions.rb file per Arcolye's answer above and tried to migrate my database again. 
Same error.
I looked at the stacktrace a bit more closely, and noticed that in fact app/admin/users.rb:25 was throwing the exception - and sure enough, that file has a dependency on my discussions table (via executing Discussion.all).
Finally, commenting out the contents of users.rb allowed me to finally migrate my database successfully.
FYI: there's a discussion here in ActiveAdmin about whether that gem should load the database when required.
So having the same problem just now. Remember to have only one model in each migration. That solved it for me. 
I came across the answer here.
I was catching the Error:
ActiveRecord::StatementInvalid: PG::UndefinedTable: ERROR:  relation "users" does not exist
LINE 8:                WHERE a.attrelid = '"users"'::regclass
It turned out to be a super easy fix. I had copied files over from an older version of the project and forgot to nest them inside of a "migrate" folder. When I did that it solved the problem for me.
it usually can happen when you use wrong type of association  between models, check for dependency destroy and has_many associations, for example: 
wrong way that can cause this trouble: 
article.rb 
 has_many :subcategories, through: :categories, dependent: :destroy
subcategory.rb
has_and_belongs_to_many :articles
right way:
article.rb 
 has_many :subcategories, through: :categories, dependent: :destroy
subcategory.rb
declare association with categories here not articles (belongs_to / has_many_and_belongs_to)
My case was also related to FactoryGirl/FactoryBot and I just had to change the definition. Replacing the constant by a string.
My env:
Rails 5.2.6
factory_bot 4.8.2
From this:
FactoryBot.define do
  factory :user, class: Admin::User do
To this:
FactoryBot.define do
  factory :user, class: 'Admin::User' do
Which is something recommended by Getting Started - Specifying the class explicitly.
        
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