Validate field using other entities

[tiurlumbangaol]

Hi, We've added validation for field to solve issue #66.

For example:

• There two entities Employee and Position
• Employee entity have position field that can only be inputted using the value from Position entity

This is how it looks like

Here is the base.json file base.json.txt

[coveralls]

Coverage decreased (-0.3%) to 79.879% when pulling 2fcc090aae73375c0a0fcb0ad634dc7ada42723d on tiurlumbangaol:master into 437bf9b1939658f40d960516e802756d7353c2be on yasuflatland-lf:master.