How to add source in GeoJSON layer using OpenLayers

Question

I am trying to add GeoJSON layer using OpenLayers which is in the PostgreSQL db. When I am running query as

select ST_AsGeoJson(ST_Transform(geom,900913)) as farmcode from farmdatadd where areatbl = 26

I am getting multiple row of MultiPolygon on which I want to add as a layer on map. But only the first array of layer is added and remaining all is not appearing.
My code is:
function addGEOJSONLayer(geojson) {
            var vectorSource = new ol.source.Vector({
                  features: new ol.format.GeoJSON().readFeatures(JSON.parse(geojson[0])),
             });
            if(geojson.length > 1) {
            console.log("here");
                for(var k = 1; k < geojson.length; k++) {
                console.log(k);
                    vectorSource.addFeature(new ol.Feature(new ol.format.GeoJSON().readFeatures(JSON.parse(geojson[k]))));
                }
            }
             
            var vectorLayer = new ol.layer.Vector({
                  source: vectorSource,
                  style: styleFeature
            });
            var ext = vectorLayer.getSource().getExtent();
            map.getView().fit(ext, map.getSize());
            map.addLayer(vectorLayer);
        }

I have done the same thing in old version of Openlayer as :
if (jsonString != "") {
        //var holdingFeature = JSON.parse(jsonString);
        var holdingFeature = jsonString;
        console.log(holdingFeature);
        console.log(holdingFeature.length);
        for (var i = 0; i < holdingFeature.length; i++) { 
            console.log(holdingFeature[i]);
            vector_layer_feature.addFeatures(geojson_format.read(holdingFeature[i]));
        }
    }
    for (var j = 0; j < vector_layer_feature.features.length; j++) {
        vector_layer_feature.features[j].style = { fillColor: "#4169E1", fillOpacity: 0.5 };
    }
    map.zoomToExtent(vector_layer_feature.getDataExtent());
    vector_layer_feature.refresh({ force: true });

Any suggestions?

geozelot · Accepted Answer

It is not clear if the passed in geojson argument is an array of rows of TEXT representing a geometry each. This is assumed in your function. Do you ever hit the "here" console log?
As mentioned in comments, better fetch a FeatureCollection (as JSONB) from the DB directly [*]:
SELECT JSONB_BUILD_OBJECT(
         'type',     'FeatureCollection',
         'features', JSONB_AGG(ST_AsGeoJSON(fc.*, 'geom_900913')::JSONB)
       )
FROM   (
  SELECT *,
         ST_Transform(geom, 900913) AS geom_900913
  FROM   farmdatadd
  -- WHERE  ST_IsValid(ST_Transform(geom, 900913))
) AS fc
;

and parse it with
var vectorSource = new ol.source.Vector({
  features: new ol.format.GeoJSON().readFeatures(featureCollection, { featureProjection: 'EPSG:900913' }),
});

Pedantic note: as per the GeoJSON specification, geometries should be stored in EPSG:4326; you can specify a dataProjection in OpenLayers (in different locations with different precedence) when defining a vector source.

[*] Here's a neat custom ST_AsFeatureCollection packed as a set of user-defined (moving) aggregates, returning JSONB.
Usage:
SELECT ST_AsFeatureCollection(t.*, 'geom') FILTER(WHERE ST_IsValid(t.geom))
FROM   <table_expression> AS t
;

Description in comments. Disclaimer: I made them.

Sanket Sardesai · Answer

Apart from directly fetching feature collection from Postgres itself, if you have an array of Polygon/MultiPolygon you can do -
ol.proj.proj4.register(proj4);// This line is optional and required if you have different source projection, here you are registering proj4 js for projection.

const polygon = new ol.Feature({
        geometry: new ol.geom.MultiPolygon(feature.geometry.coordinates),
        exampleKey: exampleValue
});

const src = 'EPSG:27700' //optional
 const dest = 'EPSG:3857' // optional
 polygon.getGeometry().transform(src, dest) // optional
 vectorSource.addFeature(polygon) // adding polygon to the existing source

here feature is a geojson feature, so you have to add the above code in the loop, and just pass the coordinates as I have mentioned above.
Fiddle: https://jsfiddle.net/amsanket22/19ztaerq/20/
If you have different projection you can use proj4 as I have shown. Also make sure you have style added for your multi polygon.

How to add source in GeoJSON layer using OpenLayers

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