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How would a police officer be lined up to work security at a bar? Would the bar ask the city or the officer? - up to dateexpiration date

I wonder a little closer to the legal aspects, such as a police officer for the working lines to help the security in a bar. How can a police officer involved specifically in this work - "The application of the bar, the officer sent to chance or not specifically bar the negotiations, particularly with a police officer in the public or is there a reason, and it depend on?