How to insert a GeoJSON polygon into a PostGIS table?

I need to insert a polygon from GeoJSON to my PostGIS table. This is how the SQL query looks like.

INSERT INTO tablename (name, polygon)
VALUES (
    'Name',
    ST_GeomFromGeoJSON(

        '{
            "type": "Polygon",
            "coordinates": [
                [7.734375,51.835777520452],
                [3.8671875,48.341646172375],
                [7.20703125,43.580390855608],
                [18.6328125,43.834526782237],
                [17.9296875,50.289339253292],
                [13.7109375,54.059387886624],
                [7.734375,51.835777520452]

            ]
        }'
    )
)

Unfortunately, I get an error message.

ERROR:  Geometry SRID (0) does not match column SRID (3857)

The GeoJSON is already in the right reference system. But this isn't specified. How do I specify the SRID in the GeoJSON? What does the GeoJSON need to look like?

Update: When I wrap the geometry created by ST_GeomFromGeoJSON with ST_SetSRID(..., 3857) it throws another error. In my view it doesn't seem that the geometry has a Z dimension.

ERROR:  Geometry has Z dimension but column does not

Answer

Taking a look at the source code of PostGIS I found out how it parses SRIDs. Here is the correct way to specify the SRID in GeoJSON.

The GeoJSON specification says that the coordinates of a polygon are an array of line strings. Therefore I had to wrap them with additional brackets.

{
    "type":"Polygon",
    "coordinates":
    [
        [
            [-91.23046875,45.460130637921],

            [-79.8046875,49.837982453085],
            [-69.08203125,43.452918893555],
            [-88.2421875,32.694865977875],
            [-91.23046875,45.460130637921]
        ]
    ],
    "crs":{"type":"name","properties":{"name":"EPSG:3857"}}
}