Added support of "Violates Foreign Key Constraint"

[amirejaz75]

• [Y ] Do only one thing
• [ Y] Non breaking API changes
• [ Y] Tested

What did this pull request do?

Updated the translator and added the support of "foreign key constraint violation". For this, this error type is needed here.

User Case Description

Lets say I have a struct RoleAssignment having RoleID as a foreign key

type RoleAssignment struct {
	RoleID      int         `gorm:"type:INTEGER;NOT NULL"`
	Locations   string      `gorm:"type:VARCHAR(255);NOT NULL"`
	SubjectID   int         `gorm:"type:INTEGER;NOT NULL"`
	SubjectType SubjectType `gorm:"type:VARCHAR(255);NOT NULL"`
	Role        Role
}

and I want to insert a record having role_id which does not exist in Role table, here gorm is throwing pgerror which should be translated to gorm error. As suggested below:

result := tx.Create(&roleAssignment)
		if result.Error != nil {
			if errors.Is(result.Error, gorm.ErrForeignKeyViolated) {
				// return Custom Error from here 
			}
		}

Pull Request for https://github.com/go-gorm created.

[amirejaz75]

@jinzhu kindly merge this PR as it's needed for go-gorm/postgres#185 (already merged). Thanks.

[amirejaz75]

@jinzhu please release new changes as these are needed in gorm/postgres. Thank you.