function m2mFromFieldName(declarationModelName) {
    return `from${declarationModelName}Id`;
}
Returns the fieldname that saves a foreign key to the
model id where the many-to-many relation was declared.

Example: `Author` => `fromAuthorId`

@param  {string} declarationModelName - the name of the model where the relation was declared
@return {string} the field name in the through model for `declarationModelName`'s foreign key.