constructor(opts) {
        const { createDatabase } = { ...ORM_DEFAULTS, ...(opts || {}) };
        this.createDatabase = createDatabase;
        this.registry = [];
        this.implicitThroughModels = [];
        this.installedFields = {};
        this.stateSelector = opts ? opts.stateSelector : null;
    }
Creates a new ORM instance.

@param {Object} [opts]
@param {Function} [opts.stateSelector] - function that given a Redux state tree
                                         will return the ORM state's subtree,
                                         e.g. `state => state.orm`
                                         (necessary if you want to use selectors)
@param {Function} [opts.createDatabase] - function that creates a database