Given the question: Information:  - A nightclub (also known as a discothèque, disco, dance club or club) is an entertainment venue and bar which serves alcoholic beverages that usually operates late into the night. A nightclub is generally distinguished from regular bars, pubs or taverns by the inclusion of a stage for live music, one or more dance floor areas and a DJ booth, where a DJ plays recorded music and where coloured lights illuminate the dance area. Another distinction is that whereas many pubs and sports bars aim at a mass market, nightclubs typically aim at a niche market of music and dancing enthusiasts and clubgoers. The upmarket nature of nightclubs can be seen in the inclusion of VIP areas in some nightclubs, for celebrities and their guests. Nightclubs are much more likely than pubs or sports bars to use bouncers to screen prospective clubgoers for entry. Some nightclub bouncers do not admit people with ripped jeans or other informal clothing as part of a dress code. The busiest nights for a nightclub are Friday and Saturday night. Most clubs or club nights cater to certain music genres.  - Opera (English plural: "operas"; Italian plural: "opere" ) is an art form in which singers and musicians perform a dramatic work combining text (libretto) and musical score, usually in a theatrical setting. In traditional opera, singers do two types of singing: recitative, a speech-inflected style and arias, a more melodic style. Opera incorporates many of the elements of spoken theatre, such as acting, scenery, and costumes and sometimes includes dance. The performance is typically given in an opera house, accompanied by an orchestra or smaller musical ensemble, which since the early 19th century has been led by a conductor.  - Nelson Ackerman Eddy (June 29, 1901  March 6, 1967) was an American singer and actor who appeared in 19 musical films during the 1930s and 1940s, as well as in opera and on the concert stage, radio, television, and in nightclubs. A classically trained baritone, he is best remembered for the eight films in which he costarred with soprano Jeanette MacDonald. He was one of the first "crossover" stars, a superstar appealing both to shrieking bobby soxers as well as opera purists, and in his heyday, he was the highest paid singer in the world.  - A baritone is a type of classical male singing voice whose vocal range lies between the bass and the tenor voice types. It is the most common male voice. Originally from the Greek , meaning "deep (or heavy) sounding", music for this voice is typically written in the range from the second F below middle C to the F above middle C (i.e. FF) in choral music, and from the second A below middle C to the A above middle C (A to A) in operatic music, but can be extended at either end. The baritone voice type is generally divided into the baryton-Martin baritone (light baritone), lyric baritone, "Kavalierbariton", Verdi baritone, dramatic baritone, "baryton-noble" baritone, and the bass-baritone.  - Bobby soxer is a 1940s sociological coinage describing the often very zealous fans of traditional pop music, in particular its creators like singer Frank Sinatra. Bobby soxers were usually teenage girls and young adult women from about 12 to 25. Fashionable adolescent girls wore poodle skirts and rolled down their socks to the ankle. In high schools and colleges, the gymnasium was often used as a dance floor; however, since street shoes and street detritus might damage the polished wood floors, the students were required to remove their shoes and flop dance in their bobby socks, hence the phrase "sock hop".  - The Love Parade is a 1929 American Pre-Code musical comedy film about the marital difficulties of Queen Louise of Sylvania (Jeanette MacDonald) and her consort, Count Alfred Renard (Maurice Chevalier). Despite his love for Louise and his promise to be an obedient husband, Count Alfred finds his role as a figurehead unbearable.  - Maurice Auguste Chevalier (September 12, 1888  January 1, 1972) was a French actor, cabaret singer and entertainer. He is perhaps best known for his signature songs, including "Louise", "Mimi", "Valentine", and "Thank Heaven for Little Girls" and for his films, including "The Love Parade" and "The Big Pond". His trademark attire was a boater hat, which he always wore on stage with a tuxedo.  - Sharon Rich (born June 11, 1953) is an author and film historian, best known for the biography "Sweethearts" about 1930s singing stars Jeanette MacDonald and Nelson Eddy. She was close friends for many years with Jeanette's older sister, actress Blossom Rock (aka Marie Blake). Along with interviewing hundreds of people, Rich had access to many collections of private letters, Eddys personal scrapbooks, diary entries, FBI files and MacDonalds unpublished autobiography. As follow up documentation to "Sweethearts", Rich has written several additional books and edited and written over seventy magazine articles. In 1995 she was awarded a Dame of Merit by the Knights of Malta for her contribution to History and Literature.  - Sweethearts : The Timeless Love Affair On screen and Off Between Jeanette MacDonald and Nelson Eddy is biographical book by Sharon Rich . First published in hardback in 1994 by Donald I. Fine , Inc. , the book covers the relationship of Jeanette MacDonald and Nelson Eddy . Rich drew heavily on Eddy 's mother , who had kept Eddy 's diaries and letters .  - Edith Marie Blossom MacDonald (August 21, 1895  January 14, 1978), known as Blossom Rock, was an American actress of stage, film and television. She was occasionally billed as Marie Blake or Blossom MacDonald. Her younger sister was screen actress and singer Jeanette MacDonald.  - One Hour with You is a 1932 American Pre-Code musical comedy film about a married couple who find themselves attracted to other people. It was produced and directed by Ernst Lubitsch "with the assistance of" George Cukor, and written by Samson Raphaelson, from the play "Only a Dream" by Lothar Schmidt. It stars Maurice Chevalier, Jeanette MacDonald and Genevieve Tobin and features Charles Ruggles and Roland Young. A French-language version, called "Une heure près de toi" was made simultaneously, with Lili Damita playing Genevieve Tobin's role.  - Love Me Tonight is a 1932 American Pre-Code musical comedy film produced and directed by Rouben Mamoulian, with music by Rodgers and Hart. It stars Maurice Chevalier as a tailor who poses as a nobleman and Jeanette MacDonald as a princess with whom he falls in love. It also stars Charles Ruggles as a penniless nobleman, along with Charles Butterworth and Myrna Loy as members of his family.  - A soprano is a type of classical female singing voice and has the highest vocal range of all voice types. The soprano's vocal range (using scientific pitch notation) is from approximately middle C (C) = 261 Hz to "high A" (A) =880 Hz in choral music, or to "soprano C" (C, two octaves above middle C) =1046 Hz or higher in operatic music. In four-part chorale style harmony, the soprano takes the highest part, which usually encompasses the melody.  The soprano voice type is generally divided into the coloratura, soubrette, lyric, spinto, and dramatic soprano. The lyric soprano is the most common female singing voice.  - Jeanette Anna MacDonald (June 18, 1903  January 14, 1965) was an American singer and actress best remembered for her musical films of the 1930s with Maurice Chevalier ("The Love Parade", "Love Me Tonight", "The Merry Widow" and "One Hour With You") and Nelson Eddy "Naughty Marietta", "Rose-Marie", and "Maytime"). During the 1930s and 1940s she starred in 29 feature films, four nominated for Best Picture Oscars ("The Love Parade", "One Hour with You", "Naughty Marietta" and "San Francisco"), and recorded extensively, earning three gold records. She later appeared in opera, concerts, radio, and television. MacDonald was one of the most influential sopranos of the 20th century, introducing opera to movie-going audiences and inspiring a generation of singers.    What entity does 'sweethearts ' has the relation 'genre' with?
The answer is:
biography