association named not found perhaps misspelled issue in rails association

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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