Add a `whereIf` method to query builder

[rohit-gohri]

Something like:

Model.query().whereIf(condition, 'id', options.id);

[hit12369]

This might be better instead, as most cases will benefit from this.

// better name?
Model.query().whereIgnoreNull('search', args.search)

// or
Model.query().where('search', args.search, {ignoreNull: true});

Above method might create bugs that are harder to spot, eg.

// might create an undefined error
Model.query().whereIf(ctx.country, 'id', ctx.country.id);

or might be harder to read

// harder to see what the condition is
Model.query().whereIf(args.subCategoryId && args.subCategoryId.length, 'Product.subCategoryId', args.subCategoryId)

[rohit-gohri]

// better name?
Model.query().whereIgnoreNull('search', args.search)

// or
Model.query().where('search', args.search, {ignoreNull: true});

These will only be helpful if the condition is the value being compared. What about if the condition is different?

Above method might create bugs that are harder to spot, eg.

// might create an undefined error
Model.query().whereIf(ctx.country, 'id', ctx.country.id);

Same goes for {ignoreNull: true}:

// might create an undefined error
Model.query().where('id', ctx.country.id, {ignoreNull: true});

[rohit-gohri]

Something like whereIgnoreNull also isn't useful for functions or raw params:

Model.query().where((q) => {
    q.where('id', options.id).orWhere('id', '<', options.val)
});

Maybe we can implement both, a general purpose whereIf that checks the condition variable and forwards rest to where:

Model.query().whereIf(condition, 'id', options.id);
Model.query().whereIf(condition2, 'id', '<', options.val);
Model.query().whereIf(condition3, (q) => {
    q.where('id', options.id).orWhere('id', '<', options.val)
});

And a whereIgnoreUndefined for simple cases

[rohit-gohri]

The simple cases can already be handled in objection through skipUndefined:
https://vincit.github.io/objection.js/api/query-builder/other-methods.html#skipundefined