ModelAdmin.prepopulated_fields generates a string with a trailing hyphen

[monotonee]

​ModelAdmin.prepopulated_fields

After concatenating the contents of all the source fields, the JavaScript behind ModelAdmin.prepopulated_fields enforces the output string's maximum length according to the max_length of the destination field, if defined. Unfortunately, the truncation operation currently occurs after converting the source string into URL-safe characters and replacing whitespace with hyphen characters. When the source string is truncated at an index immediately following a hyphen (formerly a whitespace character), the string returned by ModelAdmin.prepopulated_fields contains a single, trailing hyphen that is unnecessary, meaningless, unsightly, and possibly problematic.

Example of current behavior using URLify from ​django/contrib/admin/static/admin/js/urlify.js:

// source_string.length === 57
source_string = 'Reading Chicken Entrails For Project Completion Estimates';

// This length will result in source_string truncation to: 'Reading Chicken Entrails For Project Completion '
max_length = 48; 

// Outputs string 'reading-chicken-entrails-for-project-completion-'
// Note trailing hyphen.
console.log(URLify(source_string, max_length, False));

Example of new, desired behavior:

// source_string.length === 57
source_string = 'Reading Chicken Entrails For Project Completion Estimates';

// This length will result in source_string truncation to: 'Reading Chicken Entrails For Project Completion '
max_length = 48; 

// Outputs string 'reading-chicken-entrails-for-project-completion'
// Note omission of trailing hyphen.
console.log(URLify(source_string, max_length, False));

This issue can, of course, be solved by custom ​form field validation but, given the spirit and purpose behind ModelAdmin.prepopulated_fields (slug generation and convenience), I believe the returns are worth the effort to ensure that the JavaScript does not return trailing hyphens.