I want to be able to fill detail into libreoffice-writer (fields), that will add the data automatically to a database. Alternatively, selected data (fields) entered into a form needs to be linked to a document (Document writer) of my choice.
You can fill Writer fields with fields from a database using the Mail Merge wizard.
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Writer is not meant to be the front-end for a database. While you can somewhat do that using macros, the better way is to use Base for that, which allows you to create forms, and actually re-uses Writer code for its reports.
Lately, Writer tables became a possible data source for mail merge: see this blog post.