Delete field from target when not in source

[michaelprichardson]

From issue #8, the new feature deletes a field in the target collection docs, the deleteMissing flag defaults to false. Field needs to be in the attribute mapping to be removed for example:

collection: 'detail1',
foreignKey: 'tempId',
attributeMapping: {
    masterDetail1: 'foreignDetail1',
    masterDetail2: 'foreignDetail2',
},
deleteMissing: true,

if the after snapshot is (master):

{ masterDetail1: 'some_new_detail' }

The foreignDetail2 will be deleted from the target snapshot as masterDetail2 is not there and maps to foreignDetail2

[michaelprichardson]

Added the same feature that was added to the deleteReference, the source primary key can be specified otherwise it will be set to the default masterId:

source: {
    collection: 'master', // <-- This will append {masterId}
    // OR
    collection: 'master/{masterId}', // <-- Can be any string as in Firebase
},

[nbransby]

Do we need the deleteMissing flag? I understand that it preserves the existing behavior but Im not sure that's good thing - I cant think of a use case where you would want the last stale value left in the target when the field gets deleted in the source let alone making this the default behavior.

I believe this warrants a breaking change - and and major version number increase

[nbransby]

Want to update the version no. is this PR?

[michaelprichardson]

Yeah sure. I was going to do it in #4