def ensembl_genes(self, build='37'):
        """Return a dictionary with ensembl ids as keys and gene objects as value.

        Args:
            build(str)
        
        Returns:
            genes(dict): {<ensg_id>: gene_obj, ...}
        """
        genes = {}
        
        LOG.info("Fetching all genes")
        for gene_obj in self.hgnc_collection.find({'build':build}):
            ensg_id = gene_obj['ensembl_id']
            hgnc_id = gene_obj['hgnc_id']
            
            genes[ensg_id] = gene_obj
        
        LOG.info("Ensembl genes fetched")

        return genes