association named not found perhaps misspelled iss

Here is my controller

@post = Post.joins(:customers).select("customers.*,posts.*").find params[:id]

My post model

belongs_to :customer

My customer model

has_many :posts

And i am getting error as

Association named 'customers' was not found on Post; perhaps you misspelled it?

This is my controller output:

Processing by PostsController#show as */*
  Parameters: {"id"=>"6"}
  Post Load (0.5ms)  SELECT "posts".* FROM "posts" WHERE "posts"."id" = $1 LIMIT 1  [["id", "6"]]
Completed 500 Internal Server Error in 113ms

ActiveRecord::ConfigurationError (Association named 'customers' was not found on Post; perhaps you misspelled it?):
  app/controllers/posts_controller.rb:16:in `show'

This is a typical typo error:

@post = Post.joins(:customers).select("customers.*,posts.*").find params[:id]
# should be:
@post = Post.joins(:customer).select("customers.*,posts.*").find params[:id]
                          #^^ no plural

Because you defined the relation like this (using singular):

# Post model
belongs_to :customer

Some stuff to know:

In the joins/includes method, always use the exact same name as the relation
In the where clauses, always use the pluralized name of the relation (actually, the table's name, which is by default the model name in plural but can also be manually set)

Examples:

# Consider these relations:
User has_many :posts
Post belongs_to :user

# Usage of joins/includes & where:
User.includes(:posts).where(posts: { name: 'BlogPost #1' })
                  #^            ^
Post.joins(:user).where(users: { name: 'Little Boby Table' })
              #^^           ^

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